Could be some little bait fish is wedged somewhere in the overflow line going out the transom. Or you dont have a gasketed hatch door that locks tight. Or a live bait while swimming around in the tank is getting stuck against the over flow outlet inside the tank. All 3 things have happened to me.
All though I have a Potter hull I moved my livewell tank where yours is.
There is other possibility: if the over flow happens when current is pushing against the stern of the boat which restricts the amount of water that can travel out the transom.
Backing up can also push water up the overflow hose, or a air bubble trapped in the overflow line. Its possible the overflow hose is to small in diameter or the boat is to stern heavy.
FellowShip
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Just for the Grins
Official 23 SF Antique Classic SeaCraft Owner