That's... surprising to me. Not on policy or anything, where it was clear she was with Sanders and Warren, but I figured she'd wait it out until it was just one of them left.
From a completely cold and clinical standpoint, I also think it's a bad decision politically. If she had to pick one at this time it "should" have been Warren, IMO. Going with Sanders increases the risk of them splitting voters even more and giving Biden a bit more of a lifeline. But really I think her "best" move from (again a completely cold logical political position) was to wait until only of Warren/Sanders was left or had clearly "won" between the two.
Yeah, AOC is not this type of person and I am grateful for that. ---