From another forum:

With Exchange there are 3 possible causes:
1. Mailbox manager is set to clear out old appointments - this is applied by
policy and can affect all mailboxes or those within a group.
2. Users set up Autoarchive - this is set per user.
3. Syncing with a deceive such as a blackberry or similar mobile device. The
default for blackberry is 60 days, not sure for the other device types.