does anyone know the terrain near monterrey?
It's relatively rough around there. The first time I went there was at night in VMC. The next day when we flew out I became a true believer in our policy for following IAPs at unfamiliar airports at night! Ironically we were flying a very popular Mexican band on an extended performance tour at the time.
On a direct course to MMTO, the terrain south of Monterrey is quite mountainous though jets are normally above the highest terrain within just a few short minutes after departure. Obviously something non normal occurred and we won't know what it was for quite some time if ever.