Broke apart due to structural weakness. There is foliage in the way but otherwise looks like maybe it was topped or broke off due to natural causes in the past, and codominant side branches finally got heavy enough to tear off. Whether it was topped or not, it is an excellent demonstration of one of the reasons topping trees and then letting replacement tops grow tall afterward is a bad idea - result is the same as if it was topped and then split apart for that reason.

Best idea is to not top the tree in the first place.

No there was a reason. bboy is right, this is a classic co-dominant failure on what looks like a eucalyptus tree that was topped as a control. Eucalyptus were imported from Australia into California around 1880 on as a fast growing forest tree. Thing is they grew TOO fast and have excessively populated half the state, taking over native species. As one method of control they have been cut back severely (topped) but eucalyptus vigorously and quickly grow back this kind of weak 4 way co-dominant sprout that is not nearly as strong as the original stem. This is a major problem in California and there is no easy or cheap way to control their growth (except acts of "God" like this which was really an act of uninformed men)

Eucalyptus do have the bad habit of dropping large limbs with no warning. I know someone who was almost hit by one while golfing.

Must have been a pretty wild shot.