While appearing on talk shows in Mexico today, Alberto Del Rio noted that he cannot wrestle in the United States for a year. He admitted there was a confrontation with a WWE employee but did not go into any great detail.
That would likely mean he was found to be in breach of his WWE contract. When a WWE talent is found in breach, they are informed that they will no longer be paid additional revenue due to them past the day of the breach (including royalties due for PPV, licensing, etc.) and that as a penalty of the breach, they cannot compete in pro wrestling or MMA for a period of one year.
PWInsider.com has reached out to WWE regarding the circumstances of Del Rio's departure and his advertised appearance for AAA in Mexico this weekend, but given that it's Summerslam weekend, it's not likely we will hear back immediately.