Why do electrical light swith makes a popping sound only sometime when turned on.

More than likely there is a loose connection. This needs to be fixed immediately.

Remove the cover from the switch, watch the switch as you work the handles and see if the switch spits as the handle works. If you see arching or little shots of light, that's sparking.

Turn the breaker for this switch off. Verify with a meter or circuit tester that the power is off. Pull the switch and look at the terminals. If one is blackened then pull on the wire and see if it moves. If it does, examine the wire to see if it is copper or alum. If it is Alum, verify that the switch is rated for an alum wire. If it is, then tighten the connection and re-install the switch, test it then cover it back up.

If it is not rated for alum, then hotfoot it down to Lowe's or whoever is closest and get a switch rated for alumn.

Alum. Wire will work its way loose under a terminal screw.

I would replace the switch, as most likely the switch mechanism is weak and not opening fast, as originally designed.

The fast break action of a switch is intended to eliminate the arcing that would occur from a load.

When the switch gets old and weak, the spring does not react as fast as it did when new.