Most common is people failing RLP (and you have to fail twice to be rolled). Next is IST, but that's for people who show up out-of-shape, since you're coming off AD you should be fine since you can obviously pass a PFA. There's also falling out on Outpost (or I guess it's like Wake-Up Friday now), if they think you're too weak they can pull you out and roll you, I've seen that happen to two people. There's also academics, but again, they'll give you a retest for each. I've only ever seen one person rolled for academics.
Last thing: If you go LLD during the first week, you're rolled automatically. Which is a total PITA, saw a few people go to H for that.