Need some guidance. What is causing this.

Ok so I have this issue from time to time. I am pretty consistent at 62% hydration, 48-72 hr fermentation. I cook in a WFO at 400-420C and use semolina when stretching. Its odd because it isn’t consistent, even in the same batch of pizza dough. What am I missing? Sometimes its really bad… typically its 1.5- 2 min and def leaves a black mark on the floor of the oven.

All thoughts are helpful.