Well I'm not aeronautical engineer, but it seams when you reach a certain angle of attack the float create extra lift, That lift being so forward of CG not spread out like with normal floats act like an extra elevator, that little force has a lot of leverage.
It's not only exclusive to taildragger floats, it has to do with the fully symmetrical airfoil too,, I had a Great Planes Super Decathlon set up on Goldberg floats that never flew quite right either, very pitch sensitive. The wing would stall but the floats would still lift,, That thing finial snapped into a full stall and hit the ground,, Fixed the plane,, never put it on floats again
With my ugly stick, I just trim it at a low to medium speed and pretty much am always working the elevator stick,, it almost never flies hands off
Just fly it like you stole it,,,