I'll weigh in, since I'm headed to Berkeley in the fall. Most of my reasons below will relate to Berkeley, as I don't know much about UCLA since I didn't apply there. (However, looking back, I wish I would have applied to UCLA instead of one of my east coast schools--I'm a west coast kind of guy).
Reasons why Berkeley has risen:
1) Small program. Berkeley and Dartmouth are the smallest schools in the top 15. Dartmouth I can understand, but Cal is a big school in a big state. I think they would easily have the demand to double the size of their program. But smaller size = naturally lower admit rate, which I'm sure helps the ranking. At the same time, I think some other b-schools (Virginia?) have expanded recently.
2) Different value system. This one is hard to quantify, but I think that Berkeley has put an emphasis on topics that have become relatively more popular in the 90s and recently. Such topics would be things like entrepreneurship, socially responsible business, and management of tech. I think these things go in cycles.
3) Great admissions team. Most of the info sessions I went to were the same, but I was really excited about Berkeley after attending their info session. The two main guys are really difference-makers. I would wager that they are responsible for increasing the number of apps, which lowers the admit rate.
4) Weather/location. I think that over the past 5-10 years, San Francisco has become a relatively more popular place than some other locations, especially places not on a coast.
5) Tech boom: everything pelihu said.
6) Catch-up with parent school. I don't really have the facts to back this up, but it seems that in the past, Haas was ranked relatively lower than the university as a whole. I think this was especially the case with Haas compared to the other grad programs (vaguely remember hearing something about how Berkeley has nearly the most top 10 grad programs--but don't quote me on that). So I think it's somewhat natural for the b-school to "catch-up" to the parent (and for this same reason I think you'll see Yale's SOM rise as well).
So those are my thoughts. While I think that Berkeley is strong (and weak) in other areas, I only tried to list the factors that would have an impact on the ranking.
Also, for the record, I think that reputation is important, but rankings are not.