Failed joint at shower head

I finished plumbing a new bath/shower about 3 months ago. Just recently, I discovered water leaking into the basement. After some sleuthing, it appears that the joint between the copper pipe and the brass dog-ear at the shower head has failed. I have 2 questions:

1. How did this happen? It was definately not leaking during the entire tiling process and for the first couple months of use.

2. Is there any way to fix this without ripping the entire wall apart and redoing the joint?

Are you sure its that and not the shower arm to drop ear elbow connection?

Pretty sure. I can peek into the wall and wiggle the shower arm and see the leak. Actually, if I put some downward pressure on the shower arm, the leak stops.

Well then pretty much all you can do is open the wall and resweat the connection.

leak

How were you testing it during the "entire process"? The only way it can leak, and/or move is if it were a bad solder joint, and that should have started leaking almost immediately, unless the joint was just good enough to hold until something disturbed it. Now, you have to take the joint apart, you cannot just "resolder" it.