as to why Mullen would go the Martinez route is that Mullen isn't completely confident in his own management of the defense yet. I think he wants somebody in there that can manage every aspect of the defense and own it. Martinez is your safer bet as he is more experienced than somebody like a Diaz and has managed an SEC defense from top to bottom. If you hire somebody like a Diaz who has much less experience, you run the risk of the defense being a total disaster should the guy in over his head. Martinez may not have a lot of upside, but you know what you are going to get with the guy - which I don't think is as bad as many would make it sound to be on the message boards.
I'm not lobbying for Martinez, I'd rather see an aggressive up and coming guy head up the D to parallel Mullen's style. But, if this was Mullen's thought process, I can understand why he's doing this. And further, if this is the case, it pretty much guarantees that Mullen and Torbush had personal differences, otherwise there was no reason to tell Torbush to hit the road. Torbush = Martinez.