It means negative sequence has occured. One reason could be that you're 1 phase short
A practical example is the single phase high speed automatic reclosure, during the AR pause, when you're functioning with only 2 out of 3 ph energized, negative sequence will appear. There's no problem for OHL or transformers, but it is a very big issue for generators. An oposite rotating field will occur and the generation unit must withstand 100 or 120 Hz instead of 50 or 60 Hz. Depending on I2 magnitude, some units could withstand for 10 seconds before getting burned. That's why you have to reclose the phase or open the other two very quickly, somewhere between 2 seconds.
