there's probably a reason. I had that driven home this morning on my ride to work.
I noticed starting out that it felt a little tougher than normal to pedal, but I chalked it up to the load I was carrying (I don't normally bring my laptop, and I had a big bag of apples) and the fact that I'd been off the bike for a few days. The closer I got to work, the harder it got to pedal until I hit a point where I could hardly move without standing. Given that there was a heavy mist in the air (it wasn't really falling rain, just sort of hanging there) I didn't want to stand since sitting back down would result in a wet butt. I took advantage of a light to examine my rear wheel, thinking that the brake might be stuck. Instead I found that the wheel itself had shifted out of position, and was not so much rubbing as locked up on the chainstay.
I had ridden into down town last week to have gelato and coffee with some friends, and while I was waiting for them to show up I decided to adjust the rear wheel, because the chain was looking a little slack. I guess I didn't tighten the drive side up enough, because the commute this morning pulled the drive side forward a good chunk, resulting in my pedal problems.
The moral of this story - double check any maintenance work you do before heading out to ride in the rain.