It's probably the extra gear thing.

I was thinking maybe they don't want their water to freeze, but if you live in Atlanta that shouldn't be a problem. Maybe they have some warm liquid and want to keep it warm.

Another idea....maybe they figure they have enough crap on already, so why not wear the Camelbak? Plus, Camelbaks can get a little hot in the summer, but that doesn't matter during the winter months.

Maybe they drop water bottles because they're clumsy in gloves.

Maybe you're mistaking extra clothing and hoods for Camelbaks.