Reducing a rather complicated explanation to a couple of sentences...
The insulating layer inside a capacitor is made up of zillions of microscopic bubbles that form when voltage is initially applied at the factory.
These dissipate over time, but reform every time the capacitor is energized, within limits.
If the capacitor sits for a long time, so many of the bubbles are lost that it won't work reliably until new bubbles are created.
That's what happens when you energize the cap for a short time then let it sit, then repeat.