1. You do more miles than before, spokes MAY break sooner; they surely WON'T, though, if you're not riding.

2. It's a random thing; there's no way to really CSI this issue.

I'm tough on bikes and parts; big guy, pedal hard, like to bunnyhop rough stuff while I'm out, that sort of thing. I RARELY get more than 2-3 years out of a wheelset, no matter who made it. Also, I've had cheaper wheelsets give up spokes more quickly (but when I replace them, they NEVER break again, always a different one), as well as the issue of splitting rims. THAT doesn't seem to be a function of what I spent on the wheels, though......

Never expect things to just keep going; it's a bike, not an Energizer Bunny. Modern bikes are built with more inherent fragility, as they are made lighter for performance. Metal fatigues under stress, and less metal fatigues more quickly.