Nothing unusual "happened".

Your pushing more than the recommended power through the EDF, and the battery is dumping current to keep up with the demand.

You'll either need to step down the voltage by using a lower voltage LiPo pack, to get more time, but this will greatly decrease speed...

Or you'll need to increase the battery pack capacity.

I have a similiar setup on another plane where I use 5000mAh packs, and I get at most 3-4 minutes of flight time before I need to start bringing the plane in for a landing.

EDF's consume gobs of power...

Did you put a watt meter on your system?

Assuming your pushing 60-80A your only talking about 3-4 minutes TOPS at high draw, given the batteries you are using!