Trip on a rebound

[KevinP]

We had this play happen recently where a player had possesion of the ball off a rebound , an opposing player was laying on floor, the ball handler as he rebounded ended up straddling the player in floor, as he started his dribble to get off him the player on floor stood up causing the ball handler to stumble and fall. We called a foul on player on floor, does verticality pertain to the player on floor?