What could've caused this?

I was running my MM5700 rustler at lunch break today when it started glitching real bad and the brakes stopped working. I pulled the body with the wheels off the ground I tried the throttle it cogged real bad a couple times then spun the wheels pretty good, but still no brakes. I figured I would just put it away and worry about it after work.

I got home and threw the pack (gp3300) on my charger- the charger gave the message "no battery". I decided to put the pack back in the truck and see what was up since it was working a few hours earlier. as soon as the plugs touched the wheels spun (power switch was off) and the PCB started to smoke right by the LEDs- I pulled the plug (I hadn't even got it all the way plugged in) and was like WTF. threw in a cheapy 1500 that I knew had a good charge and nothing.

The motor is fine as it works with my other MM esc- it's obvious that the battery and ESC are toast but what do you think caused it? bad battery smoked the ESC or vice versa? bad switch? I'm at a loss