I don't think it was wholly Juan's fault. I think both Ganassi drivers are equally at fault here. Obviously, this isn't a video, but the evidence is pretty straightforward.
It's part Juan's fault because he knew he had fresher tires, a faster car, and 8 laps left to make the move and he stuck his nose under Scott and didn't back off when he saw him coming down into the corner. He should have been more patient. He may be a rookie in NASCAR, but he's a veteran driver who should have had the patience to make the move cleanly, especially against a team mate, especially against a teammate he recently won the 24 hours of Daytona with, and especially with team orders explicitly instructing him to be carefuly about doing just what he wound up doing.
It's part Scott's fault because he did cut across the race track. He left the inside open, and Juan naturally took it. His spotter should have told him, or he should have had the common sense to know once you leave the inside open the faster car trying to pass you will be there to take it. He didn't need to cut across or cut off Juan, the next corner was a left-hander that would have had him on the inside regardless, so it was a stupid move. Likewise, he knew Juan had the better car all day and was on fresher tires. He tried to hold the lead when obviously outclassed and paid the price.
Like I said, they're both responsible. Here are the screenshots:
An understandably angry Scott Pruett hits Juan Montoya after the race in Mexico.
Overlooked stories coming out of this incident:
Scott Pruett, with far fewer laps than Juan Montoya, recovered from the spin to finish fifth. That's damn impressive and Juan Pablo better be counting his lucky stars his teammate didn't have time to make it back up to him to repay the favor. Also, Denny Hamlin gave Juan Pablo Montoya an impressive run for his money at the end. Had they been on equal tires, we might be talking about Denny's second consecutive win at Mexico.