These few nights, my girl keeps flipping to tummy position while zzz. Despite us keep turning her back to sleeping on her back. She is 6 months old. Why this behavior?

Did your baby just recently picked up the skill of rolling over? I've read that it is simply the baby trying to practice what he/she has mastered (even in his/her sleep). It is likely a phase which will pass. For now, like what you have been doing, you can just check on her more frequently and roll her back onto her back when needed.