Well, its clear what caused it! The lower rudder cap was full of water! The drain hole had become clogged with dirt and debris. Both sides were clogged.
Uavionix sent me a canned email saying this will happen every time in certain aircraft designs because there is a hole in the back of the unit for baro.
They claim the unit is protected from water, but we shall see...
Lesson - CHECK YOUR LOWER RUDDER CAPS FOR BLOCKED DRAIN HOLES!
