"Bus time" is what happens when you're working on a big project and can be summarized by saying that everything takes longer than you think. Yak shaving can contribute to the effect, but it occurs even when all activities directly support the goal, simply because there turn out to be a lot more of them than you expected. Sometimes it's because new information is revealed in the process (something is missing or broken which you didn't know about before and you have to go to the store, or the bolt turns out to be a size you don't have a wrench for and you have to go to the store again, etc.) and other times it's because the description of a task is deceptively simple ("let's get the audio set up" and then you have to swap microphones and cables and run all around trying to get rid of that damn buzz). Anyway, you finally complete a one-sentence step in your overall goal and it's three hours later than it was fifteen minutes ago when you started.
The name was coined when some friends were working on getting a big coach bus which had been used for storage for a while cleaned out and operational.
This Saturday I'm going to be getting footage for a DVD project. I can sense a lot of bus time in my future.