Thanks John, that would explain it.
I guess my real question then is:
How do I properly winterize an outdoor shower with a Moen shower valve?
Inside the house, I have two shut-off valves with drain ports: a hot and a cold. The cold serves the shower and a hose bib. Outside, in addition to the shower valve and hose bib, there is a smaller gate valve about a foot below the shower valve (to drain the shower valve?).
I do have an air compressor.