Do you fly over the tropics? To me it's not so uncommon to experience heavier turbulence when crossing the equator to the tropic of Capricorn, haven't experienced turbulence bad enough to see people pushed straight through the cover of the oxygen masks but I've been in a couple of flights where I experienced big drops in a split second, thankfully never seen anyone badly injured just some poor soggy bastard that was thrown around in the bathroom and people hitting their heads on the overhead bins.

I just always wear my seatbelt per the flight attendants' instructions, "keep your seatbelts fastened while seated" is a recommendation in every flight I've ever taken in my life.