"System Failure"

Well this is a new one on me. Due to the fact that I hate the idea of RTH when the battery reaches a 5 minute warning, I have turned that off in the software. Today, while continually reassessing the Yaw Drift issue I have written about, I reached the 5 minute remaining power mark and the IOS software duly began its claxxon warning.....No problem, I was already bringing it back and was within 20 feet of the start point and flying about 50 feet high.
Then, the screen immediately fills with a "SYSTEM FAILURE" warning, all four LEDS flash red and the Phantom begins an immediate automated decent to a fast landing which ALMOST resulted in a tip over. I thought the all Red LED flashing represented low battery but as soon as the Phantom landed the steady slow flashing green returned.
I have not seen anything about SYSTEM FAILURE mode: what causes it, what the Phantom does in response, can it be bypassed etc.
Is it likely that the 5 minute power warning triggered the SYSTEM FAILURE. Any insight into this warning and programming will help.

BTW I absolutely know how lucky I was........ and very grateful!!!!!