Blew trans fluid out overflow on trans?

[troubles789]

I was driving in the mountains of TN and VA. Doing anywhere from 70-80 mph On the steep inclines straight up. There was alot of kickdowns. Was not using OD at the time do to the steep roads. I noticed a bunch of smoke coming from underneath, out of the back on one of the kickdown climbs, trans fluid even went on the rear glass and tailgate areas. Then it stopped less then a 1/8 mile. Stopped and checked everything out. Trans fluid was dripping from under the 1993 jimmy 4.3w auto. Fluid levels all checked out and everything worked fine, so I cont on my trip. It never did it again.
So what would cause this trans fluid blow out of the vent tube of the trans? Anyone else having this happen. Has only done it twice since owning this GMC. Other time was 8 years ago doing 85mph passing on a mountain. Ideas?

[JimmySLS]

My guess would be elevation and differences in air pressure..