I consider myself to not have much bias with regard to this subject seeing as I went to BYU and I am NOT MORMON.
1. Mormon's flock to dentistry because they recognize that in their religion, a family is almost mandatory (tough to get into Celestial Kingdom without one right?), so they KNOW they're going to have a family, and thus they need to provide for it. Dentistry pays great but more (maybe) importantly is that it has a short work week so they can spend time with their family.
2. There is definitely a "undertow" of competition amongst some LDS people (as every other group I suppose), however it def pushes LDS people into fields of prestige and cash flow.
There are def some schools that accept A LOT of Mormons seemingly on purpose. VCU, Oregon come to mind for sure - the dentist I shadowed went to Oregon way back when and said about 1/4 of his class was Mormon...my good friend is a LDS D1 there now and says there is still a strong presence. I got rejected from my state school (VCU) and then got in to two out of state schools, I'm not saying it's becasue a Mormon stole my seat, however it would be nice to blame it on that. The truth is LDS people make strong applicants due to the HUGE extracurricular part of the application - 2 year mission + ample semi-mandatory church related service, and the fact that they don't do the alcohol/drug game, and tend to be older and more mature, thus causing them to be "over-represented."
EDIT: My D1 class is about 3% Mormon.