Revealing mistake: During the driving scene, the background is illuminated by non-existent headlights. It's running backward on the projection screen.
Revealing mistake: During Alex's fantasy after he says, "I knew such lovely pictures", they show a bride being hanged. You can see the "bride" has a beard.
Chosen answer: The droogs didn't like how Alex was leading them, so he attacked them. It's never explained whether the plan was to set him up all along, but given that Din was ready with the bottle to smash him over the head, it seems like an opportunity to be rid of him came up and they took it.
Captain Defenestrator