That is because his hand was covering the ejection port. If one obstructs the function of the ejection process, the round can slip off the extractor and the primer can be impinged by the ejector. Rearward movement of the slide in a forceful manor can cause a kaboom if cupping the ejection port.
Having said that, racking the slide and catching the round is virtually impossible to set off a round. If it were, hunters all over the world would have done this. Bolt guns, lever guns, pump guns, even full auto. Clearing the chamber does not cause a kaboom. Only guns this does not apply to are break actions and falling/rolling blocks.
By the way, I pick up every round I find. Pull the bullets, reuse the primed case and use the bullets for practice. I've pulled thousands of rounds. That adds up to real money.