You don't get much snow/ice in your part of CA do you
Happens to me in the SDL at least once a year. I'll let someone else give you a more detailed explanation but basically your power was taking the path of least resistance so it was going to the slipping wheel vs the one that was firmly planted (This is also a good example of how diff locks would work in a situation like this)
Also if you had a compressor you might think about lowering your tire pressure first next time
Tyre Pressures and off road vehicles - YouTube