Problem was Sabonis was facing Nurkic - and lost track of the ball - if he'd boxed it would've been easily called - you can't face-guard a player on a rebound.
Here's the play in question: (start @ 9:20)
When the ball comes off the rim Nurkic has both arms wrapped around Sabonis. Then as both of them jump for the ball, Sabonis jumps straight up but Nurkic jumps forward into his space which pushes Sabonis to the side. That looks like a loose ball foul on Nurkic to me.