I've been watching a lot of foot ball through the years, but it seems like I've seen more helmets fly off during football games in the first 2-3 weeks of this year than I've seen in all previous years combined. Seeing a helmet on the ground used to be a genuinely scary experience -- I'd be afraid there'd be a head in the helmet. Now it happens every other play.
I know that players today hit harder than they used to, but I'm not sure that completely explains the sudden jump. Has there been a change in helmet design that makes them easier to come off? I guess it would make sense from a physics perspective, since the enrygy that goes toward projecting the helmet doesn't go to the (now lidless) player's head. Is the risk of injury from that hit greater than the risk that the player will be hit again without protection? I don't know.