It's a holiday so alot of schools and work places get it off. Although not everyone does which is probably why it's not the busiest holiday. But yeah any holiday is going to be crowded. Especially when people make three day weekends out of them.
We went last October and it was jam packed! There were a ton of school aged children there and while in line we chatted with one family and they said they were there for "Columbus Week" so apparently it's not just one day...