News stories of the incident suggest that the attackers are merely unruly fans, but I think McCartney got caught up in a growing problem in Mexico. Drug gangs have recently been "adopting" singers and bands in Mexico, whether the artists like it or not. Affiliation to a band is acted upon in the same way gang colors are in the U.S., with gangs devoted to specific artists violently attacking each other.
In some cases, musicians themselves are killed by "rival" artists' devotees. An artist like McCartney doesn't have to have a gang devoted to him to be attacked by other artists' gangs; he's not their guy. It's very dangerous to be a popular musician in Mexico.
AZ Central article.