So before anyone gives me shit, I have only been in HVAC for less than a year now and I'm just starting to run my own repair calls and I'm trying to figure what was going on with this system.

I went to a no cooling call tonight for a 2-year-old Trane system. It had a nest thermostat and when I got there it was giving me an error for y1 saying it could not operate the equipment. I checked the thermostat wiring and everything looked good but there was no common wire for the thermostat but there was one there so I hooked it up. That made no change. While going through and testing stuff I had the control wire going out to the condenser disconnected and turned it back on. Everything came on like normal, except for the condenser obviously. I went outside and checked everything there. No issue with the coil on the contactor, no shorts and nothing had voltage that shouldn't have voltage. I hooked everything back up and tried jumping the wires at the thermostat. It still didn't turn on but about 30 seconds after i did that the transformer blew. It did not blow the fuse and nothing else seemed to happen. If anyone has any idea what might have happened I would greatly appreciate it, I can't make sense of why that would have been connected and why it would have blown the transformer and not blow the fuse on the board.