Guess I should have looked further for the answer first:
Settings > Mail, Contacts, Calendars, scroll down to the Calendars section and change the sync option to something other than 'Events 1 month back'.
There is a setting in iTunes and possibly also in iCloud (I don't use it for my calendar) to 'not sync events older than.....'
For example if that is set to 30 days then at day 31 you iPad removes that old calendar data.
Limiting the amount of old data is a way to keep the 'other' category of the storage space under control.
You can set that calendar up for each device.