Two possibilities come to mind to explain these odd reports of "insufficient staff" :

1) Crew Duty Time Limits
This happens to airlines as the end of the month draws near. Crew members are only allowed to be on duty so many hours within a month. If during the month, there were a lot of delays, they may find themselves "out of crew hours" at the end of the month. Crew duty time runs from gate-to-gate, so long taxi in/out times burn into the available hours. And new crews can't be hired/trained just because it looks as if this month will end up being tight.
December is the worst since there's also a yearly max for duty hours.

2) New Government Tarmac Rules ??
It's now in the airline's best interest to start scrapping flights early on if there's a chance that traffic may start seriously backing up.
All it takes is a major thunderstorm to roll thru a major airline hub to disrupt things. If operations are halted, say for departing flights, yet most of the next wave of inbound flights are already enroute, then you're setting yourself up for having 2 complete banks of flights on the ground at the same time. There simply isn't enough facilities (gates etc.) nor the people to staff them. Government regulations now try run operations for the airlines and falsely assume that it's better for the passengers if the airlines started to cancel flights during "backlogged operations", rather than risk long tarmac delays. I guess if an airline doesn't have a new cancellation code to report this situation, then the code for "insufficient staff" is probably best. In such operations, the ground staff is probably worked to the max already, leaving the airline no staff to drive buses out to the tarmac to pick up passengers, deliver pizzas, dump the toilets, etc.

Steve