Do you leave the mower outside and uncovered? Do you wash the hot mower down with a water hose after using it?
I ask this because a hot transmission with minimal moisture contamination SHOULD dissipate that moisture through a properly functioning vent.
Maybe stormwater has entered the...
I hear that some people remove the trans and flip it upside down. My AT in my Civic requires removal and replacement of 2.3(?) quarts of fluid every so often, so theoretically the fluid is completely changed every three (?) services. My civic has 270k miles with no transmission problems so far...