We suspect your plumbing is suffering from a condition called "shower-rise," in which the water can’t get out of the spout as fast as it flows through the valve. This causes water to back up and come out the showerhead. There can be one of several reasons for this kind of nonsense:
- The valve could be upside down
- The length from the valve to the tub spout might be outside the 8" to 18" rule
- There is more than one 90-degree angle
- Something other than copper or galvanized pipe was used going to the spout
- But the most common reason is that something is limiting the water flow to the spout. Solder, possibly, or something lodged in the pipe. Anyway, something is blocking the flow of water to the spout, causing it to back up and go out the showerhead instead.
If this is the cause of shower-rise, you can remove the blockage by first removing the tub spout; and then by feeding a plumber’s snake or speedometer cable up through the tub drop. The act of feeding the snake or cable will cause it to twirl and loosen the blockage. Be sure to flush the valve before reinstalling the tub spout—and also check for blockage in the tub spout itself.
If that didn’t do the trick, you may need to call a plumber. After the shower had been installed and the wall has been closed, there’s not a whole lot you can do to fix the first four possibilities yourself.