Was it for money, because they felt unappreciated, because they saw too many bad things and just stopped caring (or were ordered to do said bad things and it broke them), because they didn't care in the first place and enrolled just to get the training?
The situation is certainly quite bad, but understanding why it got there in the first place could be interesting, if only to protect other countries from the same experience - ie a drug startup could consider setting shop in, say Canada, with a much larger and far less controlled border, lots of empty space and forest to run labs (etc) using the very same methods.
Reservations, crime and drug friendly cities like Winnipeg could offer them a great advantage - or if we talk about geography, sea access from Labrador to the European market could also be an advantage.
So the question is - why did it happen in Mexico?
Mexico is a needed stop into the US. From that article, "Due to U.S. interdiction successes in the Caribbean during the 1990s, Mexico has now become the single most important way-station for cocaine and heroin produced in the Andes, and is itself a major producer of marijuana and methamphetamines. The permeability of the U.S.-Mexican border allows for easy transit into the United States, and Mexico’s share of the drug trade has grown steadily over the past 15 years."
IMO, whenever you have as much poverty as Mexico, but a very wealthy neighbor, this can be a possibility.
So that explains why Mexico instead of Canada. But I'm not sure what motivated those particular soldiers to go rogue.
You might want to look into the margins on cocaine production. Either that or the person selling you your greenhouses is ripping you off.