I would just watch it for a few days.
Things pop and crack on me all the time, granted normally I'm forcing them to.
having something pop in your back/glutes when it's not expected it weird though.
You might have just been spooked and your mind led the body to not go very far.
Your mind is more powerful in pushing big weight than most people understand. You have to be in the right mindset.
Finally, I'm fairly sure that's Shawn Ray's back...it's kinda insane for most normal people to have. Pictures of members in their avatars are usually of lower quality.