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Both iCal and the Address Book typically synchronize with the framework in the background in response to changes pushed by other clients to the truth database. Take a look at this document for an overview of how this process works:
http://developer.apple.com/macosx/syncservices.html
Something flag stored in the framework is apparently 'inviting iCal to the party' each time you initiate a synchronization event using iSync. You may be able to stop this behavior simply by resetting it. To do so…
• launch iSync
• open the Preferences… pane
• press the Reset Sync History button and follow the instructions which appear
This has the effect of leaving your sync clients registered and the data stored in or on them intact, but empties the truth database and creates a condition in which you appear never to have synchronized. This, in turn, triggers a slow synchronization event the next time you initiate one with iSync.