And this has been the problem with this thread. Disregard those that actually work at the airline in question. We apparently are clueless while people at other airlines know exactly what's up.
PhilosopherPilot is exactly right. Guys couldn't get the days off they needed through the bidding, multiple drop/swap windows or trading trips. So, they called in sick. Not only did I see this a lot in my past year here, I saw it a LOT at Pinnacle. It wasn't rare there at all. It was pretty much the main way to adjust your schedule since everything was "denied due to staffing."
I don't know how prevalent it is at Spirit, United, Colgan, Delta, GoJets or anywhere else. And I won pretend to. But I can tell you it wasn't "rare" at either airline I've worked for.....