This is really strange, never heard of anything like it before. I myself on rare occasion "need" to crack my hip in arabesque in order to gain extra height but this is very distinct from what you describe.
The only thing I can think of is that it might be pulling of tendons due to a muscular imbalance or something similar. Does this popping happen after you have thoroughly warmed up or only when you hold arabesque "cold"? Also, have you maybe had hamstring problems in the past that may have left you susceptible to some sort of imbalance?
If it is not painful then it should hopefully not be anything to worry about, but as with all medically related issues if it is troubling you, it can never hurt to go to your GP and have your mind put at ease!