The answer that makes the most sense to me is that the Mexican government has incriminating pictures of Bush from his drinking days.
It doesn't seem like too big a stretch to think Bush did some heavy partying in Mexico and if Bush attracted attention to himself on a heavy drinking outing, the Mexicans would love to have some leverage over his politician father and luckily for them now Bush himself as President.
“The answer that makes the most sense to me is that the Mexican government has incriminating pictures of Bush from his drinking days.”
I feel like I’m in a scene from “Deliverence”.