Usually something against the law, but this varies depending on the country. One common mistake would be cheating - on such cases, the 'cheated' party did not antecipated the end of the relationship and can be seen as a 'vulnerable' and 'harmed' - this usally ends with some sort of alimony or wealth division forced by a judged.
Another thing that is very common is one party asking for more than they should or another not agreeing to a fair split, so they go to court and things escalates out of control.
Using your doctor-neighbor as an example is kinda hard to say, but if they had kids and the wife could prove that 1. she was not the one that wanted to end OR she had a very good reason for it and 2. she can't afford to keep raising the children on her own, stuff like that could happen.
There are some schmucks that are so deep into being idiots that they end up losing the house, the kids and has to still pay monthly fees - these ones usually has some serious shit to hide or they got caught cheating red-handed.
Also, make no mistake - someone that makes (or allows) his partner to stay at home while he brings the income and later when they break up he says he payed for all of it, is one this type of idiots i'm mentioning. What happens here is that 1. one party gave up their carreer for supporting family and now they're left with nothing; 2. with income good enough, you should be able to rebuild yourself while the other party won't.
Again, please note that this varies A LOT considering the country