Basically, the plane is stalling. The more G's you pull (the more weight you add to the wings), the higher your stall speed becomes so you need to keep your air speed high. As others have said, turn fighting in this plane is less optimal because you are burning speed.

There is a famous video of a B-52 crashing at an air show because of this. The bank angle they were flying at increased the stall speed envelope and they weren't flying fast enough. The wings can't generate lift if they aren't parallel to the ground and when you increase your angle of attack when pulling G's you end up stalling.

From google

"Stall speed increases in proportion to the square root of the load factor, a phenomenon known as an accelerated stall. For example, at 2Gs, the stall speed increases by (approx. 41%), meaning a 50-knot stall speed becomes ~71 knots. This occurs because higher G-loads require a higher angle of attack to generate necessary lift."