OMG. I just realized (and tested) that your original description can be duplicated exactly by messing around in "Channel Settings" and making only one small error. It would be a good idea to do a factory data reset of your controller to see if that will clear the problem.
Please understand that no matter how it happens, if there are unknown changes in channel settings you will experience unexpected control anomalies. The one you have noted may not be the only one. Several crashes have occurred due to unknown channel settings. Sometimes to pilots who are certain they never made any changes at all. Yet the changes were there.