Had that happen on my drag car once, never did find a cause except the weld had started to fail. Didn't suspect any problem until it puked some ATF during a pass.
It was an old Turbo Action 4400stall in a footbrake SB/904 Duster that by then was in the 10s. Converter probably had hundreds of passes on it.
Sent it to Dynamic for repair and had them raise the stall speed, and it's been fine ever since.