Ghost shifting DT shifters

Just to make things clear, no.

The cable does not go slack, it gets TIGHTER and literally pulls the shifter back round. When on the largest sprockets the cable is obviously under the highest tension, and it is here where if I try to ride up a steep hill it downshifts.
Could frame flex be the issue as this is happening on hills? Does the shifting only happen when you're stood up and putting the power down?