Had the same thing at a church job where the ground wire on the fire panel was loose and it was back feeding through the conduit. What fixed it for me was running a dedicated ground back to the main service instead of trusting the old pipe ground. Once that was solid the zone faults cleared up and stayed clear.

Wait, did you backfill behind that wall before the rain hit? Half inch is way too deep for a raked joint on something exposed like that, you basically carved gutters for the water to run down. Raked joints work fine on vertical stuff that doesnt get hammered, but a retaining wall takes all that splash and runoff. I wouldnt bother trying to tool over the top of what's there now, that mortar is already compromised and youll just end up with a weak bond that flakes off next winter. Best move is to grind out the bad spots, repoint with something a little stickier, and keep the depth around a quarter inch max. Patch the obvious washouts first and see how it holds after the next storm, but honestly you might be looking at redoing the whole face if it keeps failing.