I found out that the hydraulic fluid for the aerodynamic feather wing grid fin things near the top of the rocket booster lost hydraulic pressure in the final seconds before touchdown due to "Running out of hydraulic fluid". The engine thrust vectoring tried to compensate but it was too far out of the correct position to save.
What impresses me is how much speed it still had coming in, this thing is sliding into home plate essentially going from 60 to 0 in 2 seconds type of thing.
Good summary here at
Bad Astronomy by Phil Plait
I like how Elon Musk called it a RUD...an acronym for Rapid Unscheduled Disassembly.