Maybe I don't understand how Uber works, but why would they have accepted it to begin with?

Drivers are often assigned their next ride while they are still dropping off their current passenger(s), so if they are a good driver they won't look at the details of the next ride until they have dropped off their current passenger(s). Once they are stopped and able to safely look at their device again, they can at that point decide they don't want the next ride. At that point, the app has to play roulette to find another driver.