why does capacitor discharge when positive voltage decrease from peak that is maximum to zero, instead of charging as the supply is still positive?

 

You should post a drawing of your circuit so we can figure out why the capacitor does the impossible.

 

Because voltage in the capacitor is higher than voltage outside the capacitor. The voltage difference induces current flow from capacitor into the rest of the circuit.

 

why does capacitor discharge when positive voltage decrease from peak that is maximum to zero, instead of charging as the supply is still positive?

IF there is significant resistance to charging, the capacitor can continue charging after the peak applied voltage has passed, because the voltage across its opposing plates may still be less than the voltage applied as that voltage is falling. Charge will begin to flow out of the capacitor as soon as the applied voltage drops below the capacitor voltage. You need a diode to prevent the capacitor from discharging back to the source, if you want to capture the peak voltage.