The reason for the majority of breakdowns are guest related. Many rides have load times. Rides have to be loaded at a certain speed. If the guests fail to load in a timely manner, the ride will cascade and have to be cycled. Example - Last spring, my daughters rode Matterhorn single rider. Older daughter loaded in the first bobsled, younger daughter was a few bobsleds behind her. As older daughter was coming into the load area, she sees this family arguing over who was going to sit where. The grouper was trying to get the family to load. The ride cascaded. Younger daughter was stuck overlooking Pixie Hollow for about 40 mins....because a family couldn't follow directions. I've seen this happen on Big Thunder, Space Mountain, and on RSR also.
The MAJORITY of time when a ride goes down, it is guest related (Indy is an exception.)
FP - wdwfan has it pretty much right. You can hold a second FP for another ride after 1. 2 hours has past or 2. the time begins. You can also get a FP anytime for Buzz. And you can hold one in each park.[
Many many many years ago I accidentally broke Space Mountain trying to get my younger, wheelchair bound, sister on. She could stand up enough that the cast member let us try three times without success and then we heard the announcement that the ride was stopped. I had no idea at that time that cascading was possible since this was WAY before the internet and my friend and I had missed the section in the brochure that said Space Mountain was NOT wheel chair accessible. We felt so bad that the rest of the trip we waited through the normal lines even though we didn't have to. I didn't realize it happened often enough to have an official name. My apologies to anyone who was on Space Mountain in September of '84. We really didn't know that we could break it!