UFOs happen for me when I either start something else as a gift or just get overly excited about new fabrics or patterns. Becoming overly ambitious is also a precursor to a project becoming a UFO.
I do have one very long term UFO that was a BOM that I purchased/started back in 2003. It was definitely out of my skill level when I began it and that is one of the things that caused it to become a UFO. Now, though, I definitely have the skills to complete it and it is on the hit list for 2013! Of course, I am seriously contemplating starting another BOM that is all needle turn applique. It is destined to become a UFO. I see it now but sometimes you just have to go ahead and purchase and then work up to it.
One last thing: I found that when I stayed out of the shops, didn't troll quilting websites, including forums, I had alot less UFOs. Now that I have found a place where the interaction is more about information sharing it is not as bad. I've been on other sites, not necessarily pertaining to quilting, but other hobbies, and every week (sometimes every day) someone would post about their 'haul'. YouTube is really bad for that in the handbag and beauty communities. That can create UFOs because everyone loves the rush of planning and starting!