Posted this in another thread and think it applies here:

Why can't Payton just leave well enough alone when it comes to defense? One minute he's announcing we're switching to a 3-4 and hires an appropriate coordinator to run it. After one up and one down season he brings in Dennis Allen and we switch back to a 4-3 essentially making Rob Ryan a lame duck defensive coordinator.

Wanna know why we might suck on defense again this year? How about having a coordinator on his way out, a defensive assistant who is already affecting the scheme, and a head coach who is offensive-minded overriding both of them to call defensive plays. It's a mess folks.