It looks like when the modal is closed, it doesn't dispose of the extra HTML\iFrame it produces. On repetitive opening and closing of modals this can build up quite substantially on a single page.
First Modal Popup
Second Modal Popup
I've tried closing the model from within the modal itself and from the parent page but both leave this html in the parent to build up. This is also the case for the example on DayPilot website (where I got the screenshots). This also occurs if you reopen the same event over and over.
Development issue - If you've enabled "Browser Link" in visual studio while debugging, this can eventually stop your modal from loading altogether, this doesn't appear to happen on the DayPilot site, but maybe something to be aware of during dev.
Is there a built in function within the DayPilot libraries to avoid this? Is this even a known issue?