It can be a mix of a few things, with the bigger guys at 300+ pounds they already have extremely high blood pressure most likely. Then they get even more excited, snort ammonia, and exert themselves to the max while holding their breath.
Add in powerlifting gear, the pressure is insane, and your body has to release it somehow. I think the nosebleeds are almost a safety measure for the body, no?
I remember when I first got into powerlifting, I was pulling around 425, and decided to try a friends multiply Metal Deadlifter. I worked up to 395, and thought my head was going to blow off. It's a pretty intense feeling you have to get used to.