Why did my lipo swell like a balloon?

[Bmr4life]

I thought my gearing was safe, but maybe I was wrong. Here's the setup.

RC8T CE, 1800kv 1y motor, MMM, Turnigy 5S 25C 5000MAH lipo. Geared 18/46. 5.5" imex tires.

Top speed should be ~50mph with no tire ballooning, but I had ballooning.

Battery swelled up in ~8 mins. Esc went into protect mode. Of all days, I forgot my temp gun. Battery was hot as hell, motor was at the high end, esc on the low range of hot.