Thank you everyone for your valuable inputs. TSandM, I understand what you described as "dancing shrimp" phenomenon and have been working to avoid that. What actually happened was I had my body totally stabilized in trim and balanced, as soon as I extended my legs in the preparation of the back kick (in the loading phase), and no matter how slow I extended my legs, they floated. There was times when I made myself negative buoyant and had my torso lie flat on the bottom of the pool and tried extending the legs, and same happened. My jetfins are negative buoyant so the obvious cause of my floaty legs was air in the legs and feet. I guess the air was not enough to send me inverted to the surface as I could still remain at depth even though I was head down and legs up and as soon as I drew my legs back toward my butt, my trim returned.
Once again thank you everyone for your advices. In my next dive, I will pay more attention to air around my suit and try moving the air out of the legs section and see how it goes.
Thank you.