Your ESC is telling you it is time to stop flying.
If any single cell in your pack goes below 3 volts you stand a chance of ruining your pack.
My Pheonix esc reduces power at a preset amount (not sure what at the moment) but when I hookup my balance charger I am seeing about 3.5 volts per cell.(full charge on my balancer is 4.2 volts per cell)
This is with 1320 mah packs and near 10 minutes flight time (mostly hovering)