Before you start debugging, I recommend checking that you have the latest DayPilot Pro version.
Then, try checking your "theme" property value (or the legacy "cssClassPrefix"). Make sure that it doesn't include any spaces because that could affect the regex used to check if the class is already applied.
If that doesn't help, you can try searching the code for the following line:
This is where the calendar applies the class. This is done repeatedly in onmouseover handler. If the check performed in addClass() fails, it could be added multiple times.