It could have been that you had the car running for an extended period. The LT1 runs very hot, the fans kick on above 230 degrees maybe the solenoids in the trans got hot enough to malfunction. Or something else in the trans malfunctioned due to the heat, when they rebuilt the trans maybe they didn't replace the solenoids. So the heat and age of the parts may be a factor. I bought the Hypertech power programmer and changed my fan on temp. to on at 213 off at 205 that's with a 180 stat. The car runs a lot cooler now. Hypertech says the stock fan kick on temp is 240 on and off at 225, this is way high to begin with and should adjusted to a lower setting. Remember there are wires and electrical components that actually sit in the hot trans fluid I couldn't believe when I dropped the pan and saw those wires and connectors and other components sitting in that trans fluid. So old wiring old components that got hot just a guess but maybe that was the cause. By the way I just had my 4l60e rebuilt, runs like a gem now the pump spilt and the trans lost pressure so that burned the clutches due to excessive slippage. I was trying to drive it to the trans shop when it went all together I lost all gears and was lucky enough to roll into a parking lot. I then had to get flat bedded to the shop about 8 miles away.That was a hundred dollar bill for the tow. But it's all good now
Good luck with LT1, They are known for being bear to work on, that #8 spark plug well enough said LOL....