If I understand it right, sounds to me that you did not seal the joint properly at the wall. As the water is flowing over that T or Y, water is trickling out of the joint and making its way on the underside and down to the p-trap. So I have to ask, Did you glue that joint?
If you did glue it and it still leaks, that's a PITA repair (pain in the a..). Usually means having to cut into the wall, cut out that T or Y that is there and put in a new one.
Woodwork Safely,
Jim Barry
www.WoodchuckCanuck.com