An update: nothing ran this morning since the controller was in standby.
I found out that when Zone 2 would run, so would Zone 5 for some reason. I opened the controller and removed the cover. I unplugged the pink #5 wire and bent it so it wouldn’t touch any of the others. Put the whole thing back together, got it on the network and ran Zone 2.
Zone 5 (pink wire) started running, too.
So, I assume this means that my pink wire is touching my blue (zone 2) wire somewhere and being energized whenever the blue one is. Running Zone 5 (pink) doesn’t do anything now (which it shouldn’t, being unplugged in the controller.