Quark is up in smoke.

[BrianG]

Maybe it's possible that even though it says it's OK to run to 6s lipo, the voltage (~25v) is a little too high or maybe very close (if not over) the FETs' safe working voltage?

Or, it could simply be bad luck. There are bound to be a few released that may have a bad component or two. These ESCs have a bunch of paralleled FETs for each phase and if just one develops a short from the drain to source electrodes, then it could pass enough current to burn out a few traces rendering the whole thing useless.