@Lyman
I know you have "pulled" a post where you accepted that ICE was a factor, though stated again the inability of the A/P to maintain altitude in what appears to have been a fairly steady updraft (+/- turb) for about 25 seconds prior to A/P disconnect.
The A/P was pitching the aircraft ND in an attempt to maintain assigned ALT, and clearly from the graphics, the RVSM parameters were being exceeded. Perhaps the A/P disconnect was for that cause, and the UAS just happened along with the rising "warm/wet" air. The RTLU "latch" could only have been caused by the "speeds", and coincidentally the RVSM and UAS issues collided - time wise.