This occurred to me at least 6 times during the weekend. Game crashed either while I was assigning a weapon to the quick slot from the inventory, and also crashed while I was equipping a weapon from the quick slot. This happened when I was clicking the respective buttons (click LS and right on the dpad) really fast. When I was performing the actions slow, it went fine. When I was doing it fast that's when the game freezes happened.

In order to perform both actions (assign/equip) we press multiple buttons simultaneously. My guess is that if these buttons are pressed in the wrong sequence and/or too fast, the game doesn't like it.

I'm on the One X, if it makes any difference.