It's an innocent enough question - "Would you be willing to serve on my thesis committee?" - and an easy thing to say "Yes." to. At the time you commit yourself there is generally no work to be done in response to an affirmative answer and generally the students who ask this are those whose work you are interested in, on some level. Sure, it'll be great to help this student along and provide advice on their work as it is coming together!
On top of the desire to help the student, there is an unspoken understanding between PIs that service on one of their student's committees means that they will return this favor down the road. Depending on the project, finding committee members for some students can be a pain in the ass, so having this "debt" in your pocket is not a bad thing.
Before you know it, you've said yes to three or four people, without much thought for the consequences of this action. Then comes the end of the semester or summer and everyone thinks "OMG, I have to propose/qualify/graduate before the end/start of the semester! We need meetings! Here's 45 pages to read by next week, and can you have comments back to me?"
I have a busy travel schedule lining up for this summer where I will be gone for 4-24 day chunks, several times. It ain't pretty, but between conferences and collaborations, it is what it is. My colleagues have similar schedules, so getting three to five of us together at the same time is damn near impossible. This means that on the rare instances that several people on related thesis committees are around, those days are booked solid with committee-related activities.
The moral of the story is to remember that on the flip side of agreeing to be part of a thesis committee is a solid time commitment down the road, likely to be scheduled at a very inconvenient time. I am learning this the hard way.