Certainly more info could have been given. Field length, elevation etc.
It says that neither pilot reduced power after takeoff yet there was a decrease in performance and power was re-applied. The first thing that comes to my mind(keeping in mind that there are differences between Tiger Moths) is that the throttle friction was loose and the throttle moved aft due to it not being held forward.
But I don't know if there is a throttle friction knob on Tigers other than the one I fly.