1) The class before that ended at 20 minutes after the hour so they can finish their lecture/discussion, pack up their things and have students ask the professor/T.A. questions.
2) The class after that will start at 30 minutes after the hour so they can set up their materials, settle into seats and be ready on time.
I would say that the students are entirely capable of sharing the space for 10 minutes but I've seen two verbal catfights erupt between instructors. In the first case it was the later T.A. who started yelling at the still there T.A. for stealing space and time from her students. The second was the teacher still there, going over his time, and yelling at the later coming teacher who needed to set up special equipment before class started.
This is my last quarter at the UW and I'm already starting to ponder the experiences and revel in the memories. :*)