Whatever the hell it was it wasn't Hayden cooking his tyres
I'd mentioned this earlier with some thoughts on what may have happened.
Up to about lap 8 Hayden was with the Pedrosa group, running the same times. He was running fast enough to keep them behind him until then, they were all on the same pace. If they could have run faster at that point they'd have surely been straight past him as Stoner and Lorenzo were disappearing up the track. Around lap 8 two things happened. They got past and started to increase their lap times. His started getting slower by up to half a second or so.
Rossi on the other hand, was using Hayden's settings for the first time this weekend. My thought was that it took him some time into the race to get comfortable with them and then he started to improve and get quicker.
Hayden was on the soft front and chewed it up by about lap 10, hence the drop off in times. Rossi, I think, was on the hard front, therefore it lasted and he was able to continue pushing as he got more used to the new bike set-up, setting his fastest times in the last two laps (not quite as fast as Barbera's fastest...)
Since my first post, Hayden has confirmed he wrecked his tyre. Up until he was very happy with the feeling of the bike, said it puts heat into the tyre very quickly and he could run whatever lines he liked. The trade off he said, is the tyre went off just as the other's tyres were coming up to temperature.