Loose Spark Plug

I have a 912 ULS. Yesterday when abeam landing point I pulled back on throttle and noticed a "difference" in my engine. Landing was uneventful. This morning when doing my run up I turned off Ignition Module B and no issues. Turned off Ignition Module A and the engine began to run rough. Did the scenario twice, same results namely, no issue when Module B was turned off but an issue when Module A was turned off.

Took off cowling and checked the spark plug wire connection at each plug. When testing the connection to the top #3 plug, the plug wiggled too when I wiggled the spark plug wire. The other plugs did not do this.

My questions are: Is it possible for a plug to come loose like that? What can happen for a plug to become loose? Is it reasonable to assume that a loose plug could be the reason for the roughness when Module A is turned off?

Thank you for your thoughts. Being ignorant like I am gives me lots of opportunities to learn new things, lol.

Charlie