Are you syncing the calendar with any online service, like hotmail, AOL, MobileMe, Google, etc.?
I have a local, MobileMe, and GMail calendars in the app, and none of them do this. I'm wondering if it might be a setting on the server that keeps overriding what you do on the iPad. So, you might take a look at your default settings on the server, if this applies.
Since a restart didn't work, and if none of the above helps, there are two more things to try.
If you are syncing with a service, go to that account in Settings >> Mail, Contacts, Calendars, open the account and turn the calendar off. Let it remove you events (they should be safe on the sever), wait a bit then turn them back on.
And then there is a soft reset.
Restart and Soft Reset
iPad: Turning on/off and resetting