Hi Sukham,
You want to remember the equation for quantifying charge on a capacitor: Q = CV. If you have a capacitor of a given Capacitance (C), then C is a constant. Therefore, Q is directly proportional to V.
As a capacitor discharges and the electrons are free to complete the circuit, the Q drops. As a result V drops as well. Therefore a voltage drop occurs during discharge.
Here's a link that may help you understand the fundamentals:
https://www.khanacademy....rs/a/capacitors-article
hope this helps!