Nobody saw the first Boone fight so who knows what happened? Might have been an off balance knock down. Also amateur fights don't matter even Tua was knocked out in the amateurs in like 15 seconds and never got knocked out in the pros.
Kov has been knocked down a time or two so I would probably agree with the Kovalev answer's. We haven't seen GGG get hurt yet(At least to where it shows)