just seen a vid from the on board camera from the bike behind him, he just lent the bike over too much and lost balance. He still had hold of the bike which made it turn across the track and into Rossi and Edwards path 
It was actually the front tyre which lost grip and slid out, then started gripping again whilst he was trying to use his leg to stand it back up. Either way, he had ~170kg resting on his leg, so wasn't able to go anywhere. It's just a shame the front gripped again, otherwise he would have slid off in to the kitty litter.
Edit: Weight correction to include fuel as it was lap 2.
