Here's my twopence worth ....... from someone who has raced all his life ........
Rossi was in front, he knew Marquez was coming and he did what hundreds of racers have done before ...... he deliberately went wide with the intention of making Marquez back off (slow down) or run off the track, he stuck his knee out which is all part of making yourself as wide as possible .... at this point, its all pretty standard stuff and happens all the time especially when the following rider is pissing you off.
Marquez being Marquez, kept on the gas and decided to come through ............ at this point he is still behind so officially whatever line Rossi is taking, is his .......... Rossi stuck his knee out, which again is standard stuff to make yourself as wide as possible and intimidate the bloke behind into backing off ........ Marquez came past so close that he contacted Rossi's knee and lost the front end, Rossi's foot came off the peg with the contact.
There is no question that Marquez was at fault .......... however, Rossi, definitely looked back, of that there is no doubt, so there is also no doubt that he timed the going wide maneuver to force Marquez off the track ...... and for that Rossi gets a sanction, that's in the Rules.
Getting run off the track by faster riders when you are in their way happens all the time, its one of Marquez's best talents ..... he has run loads of other riders off the track ...........
If the rules were applied fairly, Rossi would be sanctioned for a deliberate move to force someone off the track ....... if he hadn't look back to get the timing right, he would have gotten away with it, and ........... Marquez would be sanctioned for his dangerous overtakes .... (if you overtake somebody so closely that you touch them ...... that's dangerous overtaking......... )
Add to all that ....... the media interest ........ and the fact that everyone has an opinion ........ race direction have a hard job.
............... and finally, its pretty standard etiquette in racing, that in a championship fight like this, the riders that are not in contention will get out of the way of the riders fighting for the title .......... there is no question that Marquez was not ever going to abide by this unwritten rule and he was going to teach Rossi a lesson after all the verbals that went on during qualifying.
Conclusion ............ Marquez was behaving like a 125cc rider, ie like a young kid, and Rossi was well pissed off, and he is Italian ...... under the circumstances its a wonder that no one got hurt.
I honestly believe that if Marquez had tried that with Lorenzo, Lorenzo would have done the same, I have no doubt at all that if he had done it to Crutchlow ...... he would have got the same treatment and a smack in the gob after the race ........