residual icing?
The 737-800 can accumulate residual icing on the tailplane area, (vref ice comes to mind on the NG flap 15) ? weather and temp at the time perfect for ice build up, a GA with rear trim and ice with pitch power couple could lead to a very violent pitch up and one that is difficult to recover from if you are not very quick. Crews that are use to flying in mild / hot temp zones can get rusty regarding winter ops.