dielectric slab inserted into constant charge capacitor

I derived the equation for the force $F$ acting on a dielectric of dielectric constant $K$ being inserted into a constant voltage (connected to battery) parallel plate capacitor of capacity $C$, voltage $V$, length of plate side $l$, thickness $d$ and plate area $A$=$l^2$. $$F=\frac{-V^2C(K-1)}{2l}$$ This is constant for certain values of $V,C,K$ and $l$.

The force acting in case of constant charge (disconnected from battery after charging) capacitor is, however, variable and varies inversely with the length $x$ of dielectric inserted into the capacitor. $$F=\frac{-Q^2l(K-1)}{2C(Kx+l-x)^2}$$ where $Q$ is the charge stored on the capacitor.

I wanted to know the physical explanation behind why the force is variable in the 2nd case while it is constant in the 1st case?

P.S- I know the mathematical proof but I only want to know why this happens.