RTL Trap is "Run Time Liberty Trap" and is mostly caused by some minor corruption in the device's non-volatile memory region. Check that after reset/clear nonvol that the device hasn't recreated any files in the GPX folder after the next complete boot cycle.

Some modern NT/NTU map IMG files created from .GMAP folder data using MapSource rather than MapInstall can cause shutdown errors either while actively navigating or when only in moving-map mode. If the map was loaded directly to the device using GarminExpress that wouldn't have caused your issue however.

Another cause of RTL shutdowns/reboots can be minor firmware errors. A fw downgrade to the next lowest version may remedy that, particularly if you did a firmware upgrade just before this problem arose.