I have only see this happen one time and it wasn't to me, but a friend of mine runs an electric shift valve body (same as i do) and the brake wasn't holding and the 60' fell off really bad.
It turned out that he had some junk metal in the valve body and the brake piston was not releasing all the way nor fully ingaging, even though the electronics was working. He then put a magnet in the bottom of the trans pan.
After he cleaned the valve body everything was back to normal, like i said it didn't happen to me and we both use the same trans builder, and normally get 300 passes without freshing up on 3,000lb cars.
O'h and we have to push the trans brake button to back up.
Just a suggestion,
Zip.