TL;DR version: Of the potential risk factors listed, pregnacy is the only one for which there is some systematic evidence linking it to this type of "sneeze pee" incontinence, called stress incontinence in medical terms. The other putative risk factors, "too much sex" or STDs aren't backed by evidence (or even discussed) in the reviews of stress incontinence that I've looked at.
It's possible that some Twitter commenters have confused frequent urination (which is a symptom of gonorrhea) with incontinence (loss of control). Frequent urination is linked to urge incontinence in people with overactive bladder, but that's a different story.
The Wikipedia page about urinary incontinence (UI) you link to has a link to another about stress incontinence (SUI), which has this pathophysiology/definition:
the loss of small amounts of urine associated with coughing, laughing, sneezing, exercising or other movements that increase intra-abdominal pressure and thus increase pressure on the bladder.
That Wikipedia page doesn't say much about risk factors though, except for obesity... However there are several review papers in pubmed which discuss these.
A 2004 review said in its abstract:
Aging, obesity, and smoking appear to have consistent causal relationships with the condition, whereas the roles of pregnancy and childbirth remain controversial.
A 2011 review's abstract lists the following as common risk factors; i've added some snippets from the review itself where relevant:
- obesity, diabetes: "Incidence of new-onset SUI was related to
obesity in longitudinal cohort studies [...], supporting a
causal relationship." For diabetes, the association with SUI rather than with UI in general is less clear, studies had contradictory findings.
age: "The relative
contribution of each type of incontinence changes with age,
with SUI being predominant in young and middle-aged
women. Urge (UUI) and mixed incontinence are predominant
in older wom. [...] As women advanced in age, they were more likely to have
SUI and UUI in combination."
pregnancy, delivery, and pelvic floor surgery: "Several studies have reported a high prevalence
of female SUI during pregnancy, ranging between 20% and
60%; in particular, de novo occurrence is more frequent in
nulliparous women. [...] Incontinence during pregnancy has been shown to be a
good predictor of postpartum persistent incontinence or of
SUI recurrence 5 years after delivery [...]. Nevertheless, the
most important factor associated with the occurrence of SUI
in young women is undoubtedly childbirth. In particular,
some risk factors associated with vaginal delivery have been
described as strongly related with the development of UI." (Emphasis mine)
- genetic: huge variance in reported results for genetic factors of SUI from just 1.5% (2011 study) to 40% (2008 study).
- hormonal changes (unclear evidence)
So, Jameson's own explanation points to one of the more common risk factors (pregnancy)... even though not all reviews agree that the link is convincing. The other Twitter explanations point out to even less discussed causes (STDs, "too much sex"), but that doesn't necessarily mean they are false. It's impossible to diagnose someone post-hoc and say "this caused that in subject X" unless you can remove "this" and obtain a cure/reduction for "that".
Neither of the reviews I mentioned above discuss STDs (or too much sex, or even sex work) as a possible cause... which doesn't mean it can't be one, just that probably it isn't a common one nowadays in the general population. I tried to find some studies/data on SUI in sex workers, but insofar, no luck with that.
There isn't a whole lot of systematic epidemiological data about STDs in the US porn industry, but what is there is pretty daming. From a 2015 review:
A study conducted by the
renamed Los Angeles County Department of Public Health
(LACDPH) [ref9] found that between 2004 and 2007, there were
a total of 2,633 chlamydia and/or gonorrhea cases reported
among 1,849 adult film performers in Los Angeles County, of
which 72 percent of the cases were among females. This study
also found a 26 percent rate of reinfection within one year
among performers. In a 2012 UCLA study, nearly one-third
of performers tested positive for chlamydia and/or gonorrhea
despite the adult film industry’s testing standards. [ref10]
In (my) comparison, looking at the 2016 CDC data for incidence among the general population, chlamydia and/or gonorrhea account for 497+146 = 643 cases per 100,000 people, so a prevalence of (new cases) of only 0.64% in the general population. So the incidence in the porn industry appears way over the average for the general population.
And finally emedicinehealth.com notes that some STDs (AIDS, neurosyphilis, and genital herpes affecting the perineal area (perineal neurosyphilis)) as a cause for overflow incontinence (which "occurs because the bladder is too full and urine passively leaks or overflows through the urinary sphincter"), but it doesn't say anything about the link between stress incontince [which it does discuss] and STDs.
So, despite the fact that STDs seem much more prevalent among the US porn industry workers (than in the general population), it's still not clear that porn work is a risk factor for SUI... because while some forms of incontinence are caused by some STDs, I couldn't find a source discussing the association of STDs with SUI.
My guess is that the Twitter comments pointing to "the clap" did so because ghonorrea symptoms include frequent urination, but the latter is not the same thing as SUI.
As for the hypothesis that (too much) sex is causing SUI, coital incontinence (CI) probably deserves a mention in this context, although "too much" (sex) means something else here, namely that in women predisposed to penetrative CI, deep penetration may trigger it. Penetrative CI is associated with (other forms of) SUI more than the orgasmic CI variant is; the etiology and pathophysiology of orgasmic CI are more controversial. The (previously linked) review I in which I found this info, doesn't mention the frequency of sexual encounters as a factor for either form of CI (or for SUI). So it's probably fair to assume there's no known link.