It's probably one of those "smart" smoke detectors.
We had a similar problem at RIT years ago. In the old-style dorms, the smoke detectors were outdated and extremely sensitive. I actually set one off by accident cuz I took a hot steamy shower. When I finished with the shower, I opened the bathroom door and the steam came out... ALARM!
Luckily, only the RAs, myself, my ex-girlfriend, and a few other people were still around (my ex-girlfriend was moving to another dorm, so she got to stay a bit longer and everyone had left for the end of the school year). So, I wasn't as badily embarassed as I would be if everyone was still there.
Anyway, they renovated that dorm 6 months later and installed better smoke detectors. This time, they're adjustable through the computer. They had them adjusted higher for those who live on non-smoking floors and lower for those who lived on smoking floor. Now, all are set to higher since the dorms are now all non-smoking.