If you went back far enough in the archives, a few years ago I posted about how strange it seemed that the amount of helmets popping off over the first couple weeks of the season seemed to have skyrocketed, and it's been that way ever since. A friend and I talked about it back then (must have been about 5 years ago) and came up with two ideas.
The first, and more likely, has been mentioned already; players don't think it's cool to wear their safety gear (a Kelso's helmet type of thing) so they're not strapping up properly.
The second, and admittedly a little out there, is that players aren't wearing their gear properly because helmets popping off gives them attention. It implies a big hit that the player brushes off, the camera focuses on them, and not only that, but their faces (w/out helmet) are plastered on national TV (how cool is that!). We mainly talked about this because we noticed that as these helmets came off, the players took forever to put them back on. They'd walk around with the helmets in their hands until it was time to start lining up for the next play.
Anyway, whatever the reason, there was definitely a huge increase in the quantity of helmets coming off. IMO, as players realize this is hurting them, this rule will only occasionally be needed.