No, an out-of-office response would not be considered a bounce-back. Bounce backs are delivery failures, which could be caused by a typo, incorrect email address, refused mail……things along those lines. These generally come with a message explaining why they were bounced back, as well as an error code (which you can look up to confirm the cause)
I would say triggering of spammy title and not working properly on your subject lines. I am quite there was a list shared by someone here about words which shoutout as spam.