When you were disconnecting the front A/C , did you remove a plug ; or two ;similar to a phone wire from the unit ?
There is kind of an odd arrangement in dual A/C , propane furnace controls .
The front A/C is the master unit and the thermostat is hooked directly to it , the master sends control signals to the rear A/C and/or the furnace . So as you disconnected the wiring the A/C board sent a 12 volt start up signal to the furnace .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.