Weird...Troopers have the standard line through the radiator, that serves to preheat and cool the fluid depending on which fluid is hotter at the time. There is also an AT Temp idiot light, I believe. That didn't light up, I take it?

Info I have read says that heat-related failures on the 4L30E are rare. That didn't stop me from putting a tranny cooler on mine, though.

One stray thought: Did you get water in the fluid (do any fording lately)? Maybe if the fluid had water in it and got hot enough the water would expand and force its way out the breather (this is just a brainstorm on my part, not sure if it can even happen that way).