Does it do it all the time or just after you've flown it for a while?

If it just starts doing it after you've flown for a few minutes you might just have the LVC set to high on your ESC. The LVC (low voltage cutoff) should be set for no less than 6 volts (for the stock 2s pack).

If it is constantly doing then either your ESC might be messed up or you've got a bad connection somewhere. Check ALL of the connections, make sure they're all soldered good and didnt get a cold joint, if needed go back and resolder all of them and try it again.