Amen to Magnum's comment on the worst possible position of the Tx and Rx antennaes.
In fact, this is the orientation that I use to do a full power range check, which just makes it worse with the vibration added. This is about the best test that you can run for range checking at the field. It should be done before any maiden flight and before the first flight of the day.
Wet ground has been a source of problems for radio performance since R/C flying began. Add to that a model sitting on the wet ground with the engine running, and you have a strong likelihood that the reciever was getting a lot of false Tx signals from the reflectivity of the wet ground back to the Rx antennae.
As has already been mentioned, consider yourself lucky and heed the lessons learned. Sharing them here may have saved countless other planes. Thanks!!