Marquez is coming apart at the seams, he's completely unnerved by Rossi. You could see it in the French GP, he's terrified of getting close to Rossi (watch the aerial shot of the one time they came close to one another, Marquez gets right off the track to avoid contact). He made a lot of mistakes in that race. The announcers tried to chalk it up to a brake problem but his qualifying and his unprofessional behaviour for the upcoming race at Mugello tells the tale.
Marquez has got to calm down and let the pace open up and settle itself. He pushes way too hard early on, and loses his head. Rossi's performance in France was astonishing, setting the lap record, letting Lorenzo win to take 2nd and stay way ahead in points, and riding a very clean and perfect race. Rossi has a way of making himself very "large and in charge" on the motorcycle. He is big for a racer and after Marquez knocked himself down by hitting Rossi's rear wheel in Spain, he's very wary of getting anywhere near Rossi.
This is a great season for GP, one of the best maybe.