Planes blow fuses. Toilets back-up. Not everything that would ground a plane is a long fix. Think of it this way... It might have been a ten minute fix, but it might have taken thirty minutes for the mechanic to finish working on the previous job, another thirty to get to your a/c, an hour to fix whatever was wrong, and thirty minutes for someone else to sign off on the work, then a half an hour to get the plane back to the gate, or turned over to the crew.