Turbine D:
Good possibilities, good questions. And I agree the burner usage should not affect the LPT.
However, if the A/B fuel or nozzle schedules were not quite right, there could be a LP rotor rollback which might put the blade into an unwanted vibration region. I have observed this rollback on older engines.
If Pratt's control loops do not provide this protection - well, that's just speculation on my part.