I was wrong. The Audiobus apps seem to change other audio app's blocksize in general.
I could get the updated buffersize in (void)soundStreamRequested and (void)soundStreamReceived in ofxiOSSoundStreamDelegate.mm.
And I could check it updates to a new buffersize depending on other audiobus compatible app's buffersize.(if other app is running).
By fixing the buffersize in the two functions, I could prevent my app from crashing at least but it seems that buffersize is still getting affected by other audiobus apps although I don't even use audiobus API at all.
I think I'm going to try changing ofxPD's blocksize dynamically depending on the updated buffersize. I would appreciate if anyone can give me any advice though.