A few reasons - they saw USA influence growing to the extent of fixing Ukraine elections, they saw Russians in Donbas being slaughtered, they felt that the 2014 agreement was pooped on, they saw NATO expanding, and they felt no one was listening. But that's just an opinion, of course. Russians are a very stubborn people - they will never give up - and that is the problem as they are ideologically motivated. They simply can't stand being bullied and manipulated.