Probably because you are not being interrupted by other coworkers and because you may feel that if you have to be there, then you might as well be productive. At least, that's how I often feel when I work weekends.

I thinking it's because you're choosing to get things done, rather than having them handed to you by others.

It actually wasn't more or less hectic than any other day, really, but I will say there are times when the interruptions work for me, and sometimes they don't.

I think part of it was that there weren't as many co-workers around looking over my shoulder, so I felt comfortable for a change.

Part of it was also the fact that by doing the work I was doing it made my job for the day easier. Hooray for selfish motives! :)

Being an extrovert is also a good point. I think it'll get better when I move to the new job.