It might be because you changed the difficulty level, but I can't say for sure because I've never changed that in the middle of a game. I think a lot is just luck. Some villagers are naturally long-lived.

I try to make most of my villagers train as farmers third. Although they can get injured researching and building, farming seems to be much healthier. Once they get to Elder, I'll uncheck the prefernce box and let them do whatever they want.