I have a 12 yr old too, and I know exactly what you are talking about. I think it is alot of what the previous poster said about "they feel safest with you" and it's like they are 2 yrs old all over again, lol.
When my husband is present, and my normally very well behaved and well mannered and loved by everyone in the world son mutters under his breath at me or says something in a disrespectful manner, my husband gives him the "voice" and says something like "DON'T you talk to your mother like that. " He doesn't even have to tell him before my son is apologizing to me.
But when husband is NOT present... I tend to (as all mothers do I think at one time or another) fall into the trap of arguing with him. Like he's an equal almost. (Don't be so grumpy. I'm not being grumpy. Why else would you be so rude? --- you know.. that whole thing...).... I have to catch MYSELF and get the "dad" tone and say "I'm your MOTHER. DON'T talk to me in that tone." or something. That always seems to work much better.
I also like the idea of the other mom's of push-ups. They do that at his karate school if someone is (blatantly) disrespectful in class sometimes. "Give me 10." and if they back talk that.. "Make it 20." I'm going to start using that myself I think.
Hang in there. <<Hugs>>