They might be doing that, of course, but given human nature, I'm sure we'd see those stories in either case. Uber has a lot of drivers, and some of those drivers are bound to get unlucky and have this happen to them by pure chance. Some of them will get very unlucky, and have it happen repeatedly. Those drivers will understandably think that something shady is going on. And someone who gets struck by lightning five times might start to think that the sky has it in for them.

There's also a pretty obvious mechanism for selection bias here regarding which type of situation we're going to hear about. Few people are going to write up a post saying "I got close to my bonus, and then I got more rides, and I got my bonus, and it was fine", whereas people who feel like they're getting screwed tend to be very vocal. So the existence of these stories tells us next to nothing.