Ground fault

That'll depend on the current it draws won't it ?

Speculation about that gets us nowhere. The OP needs to measure the current draw. Guesses about it are pointless.

Since when ? And what would be the mechanism by which the load is reduced ?

Graham

draw. Guesses about

Very true

Centrifugal pumps will often produce less load on the motor when there is a restriction on the input or output. there will be less mass flow through the pump, and it will rev up even though it is pumping less "stuff". The torque from the motor (and amp draw) will decrease. It is not intuitively obvious, and is the opposite behavior to a positive displacement pump, which will usually slow down with a flow restriction.

Take a shop vac and put your hand over the hose, you will hear it speed up a lot.