This may sounds funny but when I searched about the question over the Internet, I found an overall report which stated as a conclusion:
When the heart stops forcing the blood around the body, it pools in whatever area is lowest. Sometimes people die standing up and sometimes people die lying face down. I think everyone here has enough spatial reasoning to understand what kind of blood pooling that would encourage. Meanwhile, for all that talk of relaxing muscles after death, it doesn't last forever. Certain types of muscle cells are activated by calcium ions. After activation, the cells expend energy putting the calcium ions back outside the cell. After death, the membranes become more permeable to calcium and the cells don't expend as much energy to push the ions out, so the muscles contract. This does lead to rigor mortis and can lead to ejaculation. It's real. It happens
I would like to clarify that whether this information is true. Are there any research papers or any other things to prove this?