Torque converter drain-back. Only does it when it sits overnight, I bet.
Putting it in neutral lets the pump fill the converter back up with oil. Performance valve bodies have a provision to prevent it.
Change the transmission fluid and filter. And do it often. ATF+4/type 7176 fluid is about a 15,000 mile fluid, tops, in any hills, towing, extreme temps, etc.
If you do it yourself, you'll need 7 quarts and a set of Allen or Torx wrenches to drop the filter. The pan has to come off to drain it.