At first look, this seems to be a fumble:
Joshua Cribbs should've called for a fair catch here. Because he didn't, he was fair game, and Omar Bolden hit him at the exact moment he caught the ball. That's perfectly legal.
After that, Cribbs was writhing in pain on the ground as a loose ball lay right next to him, which looked like a fumble and a turnover as a slew of Broncos came forward to recover it. But after review, the referee said that Cribbs was down by contact. It's a confusing ruling -- it certainly looks like the ball is moving around before he hits the ground, and since the ruling on the field was a fumble, the refs needed conclusive evidence to overturn it.
But one could see how the officials ruled the ground forced the fumble.
The Broncos didn't let the surprising call get them down -- they picked off Andrew Luck a few plays later.