Michael Hutchinson, the author of The Book of Floating - Exploring the Private Sea explores the question. He came up with Seven Theories of Floating.
1. The Antigravity Explanation
2. The Brain Wave Explanation
3. The Left/Right Brain Explanation
4. The Three Brain Explanation
5. The Neurochemical Explanation
6. The Biofeedback Explanation
7. The Homeostasis Explanation
I hope you find this information interesting and helpful.