Brian Whatcott wrote:

Sounds plausible to me. A direct path from unexpected rain at high
altitude (warm massive updraft in cu-nim?) to ice to frozen pitots to
loss of rudder limiting.

This meteorologist suggests that it would be highly unlikely for the Air
France flight to have encountered rain or even supercooled water. He also
suggests that the cause, if any, would be from descending air warming
rather than an updraft:

http://www.weathergraphics.com/tim/af447/

Then all it takes is injudicious heavy rudder in heavy turbulence
and you are in deep do-do....

Why would they be using rudder at cruising speed? Further, according to
the discussion in some of the other groups, the rudder limiter is still
effective in alternate law. It supposedly clamps the limit at where it was
when the shift was made from normal to alternate law.