I was thinking that if a roller touched the frog to a side rail for a split second, it would cause the short.
Sometimes our layouts change. When spring comes and the snow melts outside, the humidity goes up. That can cause the wood to swell and tracks to shift. That exact scenario caused one of my plaster bridge ends to crack from the wood hitting and squeezing the bridge. Maybe that caused your rails to change just enough to create an issue??
