Your spinal erectors are weak or are being overtrained. I used to have this problem from dead lifting and squatting in the same week. Now I just dead lift once a month. Also notice I get this problem when wearing a belt too much. I believe it causes your core muscles to atrophie. Or if it's not that it could be a muscular imbalance.
What I did was stopped wearing a belt completely. On everything. Wouldn't wven bring it to the gym. Sucked at first, bent rows standing barbell curls hell even incline bench presses were making my back tight but after a few months of pushing threw it, it finally stopped I put a belt back on my heaviest squat set and boom felt like somebody was spotting me. Helping me threw the set. It was great so now I only use a belt when going past a certain weight on excersises. Hope this helps