you have failed to mention exactly when you did your back day. many times i have not felt the effects of a good workout until the second day. it is possible for your back to stay tense until you work your chest.(your back muscles must relax to allow full range of motion by your chest muscles). that is when you would begin to feel the soreness.
#2
lats stabilize on pressing exercises. however, they work a little harder if you're you have your back arched during the press, or decline benches(which he said that he did), and dumbell presses (which he also said he did). either of these could be be the reason from what i have heard.