What caused this? Is that a joke? You(or somebody) ran an engine for 285 hours that should have had the sc rebuilt at 100 hours, that's what caused it and I'm surprised it lasted that long.
The ceramic washers have busted and the pieces are floating around in your engine and most likely will ruin your engine if not cleaned out properly. In your pic, see the gap? That's where they WERE. The washers you see are the spring washers and those aren't the ones people are referring to. The ceramic slip washers were on each side of the gear.
You may not want to hear my suggestion but I think the engine needs to come out and be completely gone through to ensure all ceramic fragments are removed. I just rebuilt a 4tec that they didn't clean out all the ceramic washers very well and it took out a main bearing which then caused a connecting rod to break and get thrown through the side of the block, it took 22 hours after the sc rebuild. At the very least, you need to pull the pto housing and front oil pump to clean and check the screens as well as the condition of the oil pumps for scarring. Then decide the next action.
How long did it run after the sc failed? This will usually determine how much damage is done and it doesn't take long.