Until recent years the hadra was totally alien to the muslims of the sub-continent. When students from the region were starting to travel to the mid-east to sit with the ulama of the arab world, they at one point or other were witnesses to something which they had no knowledge of. Thus, getting confused about how this singing and dancing could have anything to do with islam.
Shaykh Nuh Ha Mim Keller writes; I was one of those who asked our sheikh about the relation of the hadra to the shari‘a or “Sacred Law” which is the guiding light of our tariqa. As Muslims, our submission to the law is total, and there are no thoughts or opinions after legally answering the question “Does the hadra agree with orthodox Islam?”
see also; the legality of raqs and this and The Whirling Dervishes
The Sunni Voice 