What was that crackle?

I got onto a rowing machine at the college gym, strapped my feet into position, and then realised the person using it before me had left the handlebar up. I made a statement about this so the student rowing next to me heard, and as I was making an deliberate futile attempt at displaying how I could not reach the handlebar, the student rowing on my left gave me a push forward on the middle left of my back without warning. I was already leaning forward and felt (not heard) two crackles to the left of my lower back in the hipline area. I’ve never felt crackles there before, but it did not hurt. What could it possibly have been, just a ligament ping over a ligament and then back again? Or something serious?