Looks like either it started tumbling before it was intended, or the second stage failed to separate as the booster intentionally flipped for its boostback burn

Looks like either it started tumbling before it was intended, or the second stage failed to separate as the booster intentionally flipped for its boostback burn. With fins up top, the combined Starship and Booster stack is sort of like a dart flying fins-first. In theory controllable with thrust vectoring, but with some inherent instability.