Beavers are an unforgiving aircraft when stalled with a heavy load.
A climb out with flap being retracted after take off which would have been considerable to get out of the water at that load. Leaving the flap movement lever in the up position is a bad habit after retracting flap.
If the aircraft still had climb flap or more out and a steep turn attempted then if flap was pumped to tighten the turn radius (Common practice in a Beaver) with the flap movement lever in the up position then the aircraft would have departed from controlled flight abruptly.
The Beaver in normal configuration has two levers, one controlled the direction of movement, the other pumps the hydraulic pump for flap movement either being up or down depending on where the smaller lever is positioned.