I was there... basically, the organization behind this festival was awful. Those same promoters did a festival last year where many bands cancelled due to poor conditions both in safety and production. People got stuck in the middle of nowhere because the buses included in the ticket left for some 'undisclosed reasons' (the festival was held in some golf club many miles outside Mexico City).
What you see is the result of the same mediocre company: Live Talent.
All day long the venue was plagued with audio issues, more evidently the main stage. Nergal from Behemoth stated his dissapointment right after they finished their set. Evanescence were due around 9 pm at the main stage, but those same problems with the audio delayed the show between 40 mins to 1 hour. Around the same time, a barricade dividing the general admittance zone from a 'VIP' zone fell down, so the people started coming through. Both security personnel as well as the staff were overrun by hundreds of people coming through, albeit in a totally 'peaceful' manner. It was some guys at the 'VIP' zone that got pissed at the people who 'payed less' that incited some fights, thus provoking a longer delay (Evanescence was ready to play at this time).
Live Talent went completely radio silent for about 4 hours, not any single message over the PA to tell the people about the fallen barricade and that once fixed, the concert would go. Obviously, people didn't knew what was happening and at the same time, the screens were still showing Evanescence and Slipknot 'coming next'. About 1 am the PA announced the cancellation and that's when a very small group next to the stage started the riot, actually the majority of the crowd just started to leave the venue.
If it wasn't for the company's awful way to handle the situation, that barricade would be fixed, the concert would be continued and lastly, riots avoided. I completely agree those fuckers that raided the stage and burnt Evanescence's gear should be prosecuted, but it only showed once again how mediocre the staff was as to handle the situation. It wasn't 'hundreds', much less 'thousands' of people rioting as some media tends to over-exaggerate, just a pack of assholes that deserve serious punishment.