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Thanks for asking. Sorry the reply took longer than usual.
Remote Buddy only switches Behaviours if it receives an external event from OS X like "frontmost application changed" or "application terminated". In that case, it asks OS X which application is currently the active one and looks for the Behaviour responsible for that application. Only if no Behaviour for the active application can be found does it start looking for applications running in the background (like iTunes) that the user might want to control.
So, on its own, Remote Buddy will never change the Behaviour.
What's likely happening on your Mac is that you have another application (including background apps and apps without an icon in the dock) running (or scheduled to run at specific intervals) that becomes the active application (=> which triggers the Behaviour change) while you're watching the DVD and that doesn't return the "active application" status to DVD Player (=> which in turn would trigger a return to the DVD Player Behaviour).
Best regards,
Felix Schwarz