Similarly to what DA said, I think it is the concept of being told, "I'm doing it right, therefore, you are doing it wrong". There are numerous DIR divers that go out of their way to *not* say this, but there's certainly a group that does.
It is incorrect to state opinion as facts, and that is mostly what DIR does. Within their system, there is generally a single methodology at work, whether it be buddy system, rigs, dive protocols, etc. These are good systems, and work well in most cases for those that adopt them. But rather than just say, "You are not DIR if you do x, y or z", it more often comes across that you are doing it wrong.
Funny that Mike F would say, "In science we often find only one truth in any one given subject. There may be multiple theories but in the end only one (if any) is true."
Historically, many proofs to which there was no valid arguement against at the time of their "proving" have been overturned. Science often submits something as a proof, only to find that the proof was flawed.
So, the day when DIR puts themselves forward as "a way of diving focused on safety and building on a solid foundation of skills that require a complete commitment to the holistic nature of DIR", there will be alot less negative talk than today, when the statement is more to the tune that since I'm right, everyone else must be wrong...