In the north at least these tend to happen at the start of rainy season. I think growth from the previous year accumulates and then dries out in the winter/summer and when the first rains come branches come down.
In fact just this evening the power has flickered on and off a few times for 10-20 minutes. Must have been a storm somewhere and they're working to fix downed lines. We're now getting some summer storms and the first rains in 5 months and I've seeing the broken branches all over.