more info
The main reason for the split seems to have been major disagreements between UAL and MXA regarding routes. It seems that MXA may team up with AA & co.
It is also noteworthy that MXA has been sold to Slim and Salinas (the former president of Mexico and part owner of TAESA) and a third individual. Aeromexico seems to get nervous now since they have not been able to find a buyer.