Is this related to the recent increase in available close votes per user?
Without a doubt.
Is this intentional?
Sure, questions don't get into the review queue by winning a lottery. The votes to close are cast with an intent, which is to close the question.
Were all these questions in the system already, waiting for months for review?
No, Math.SE does not have a long backlog like Stack Overflow. Questions get reviewed within several hours.
are these simply because there are a great number of people using their 50 votes per day on old questions?
It does not take a great number of people. A small, but nonzero number of people is enough.