Originally Posted by
F-16GUY
Could it be that they encountered roll reversal due to being supersonic or transsonic? This will explain why the aircraft rolls opposite the command roll.
A quick calc of IAS Vs altitude tells(wihout temperature corrections made) Mach topped about 0.95 around FL260. It was quite cold so I guess it came closer to transonic at lower alt, despite a FL-to-CAS-calc dont say so.