Think of it this way: From mid-November to the end of December, there are two major US holidays where most employees will take one if not two weeks of vacation. Onboarding is an issue because you won't always have a full team in place to support the new hire. Vacation is an issue because new employees generally don't get any accrued vacation time, so you end up with idle, directionless, new employees for almost half the period. It's also easier to hide budget issues if you don't hire someone who isn't going to be useful anyway. It's just easier for everyone if you shut it down in November and start again fresh in January.
Yes, it is pretty common for hiring to pick up in January. Usually there are approved number of hires approved and they run out toward the end of the year and then many teams stop trying to hire until new headcounts are approved for the next January.
