Perhaps it's the wind of his passing that causes it.

There is no wind.

Or it might have been an illusion; depth is hard to perceive when the mist is smothering drowning stealing hiding enshrouding everything.

The loud CREAK is the only warning the would-be heroes have before two of the tree's branches crash down in thunderous fury on each side of its dark child, deflecting the brightness of beams and blasts from their intended target.