stuck lower belt?

While attempting to use Ellis' max speed tuning macro here:
https://ellis3dp.com/Print-Tuning-Guide/articles/determining_max_speeds_accels.html
at an acceleration of 27500 the lower belt jammed and I had to do an emergency stop. I was only able to resolve the jam by powering off the printer and moving the belt, gantry and print head by hand until it came loose again. I'm really uncertain what caused this situation and am hoping someone can explain what I did wrong before I resume tuning.

The belt was extremely tight but not completely unmovable. I looked at all the idler pulleys that I could see around the printer and didn't notice the belt being off any of them. The only thing I can figure is that maybe somehow something happened to the servo that resulted in it not turning freely for a while. Any thoughts on what I did and how to make sure I don't do it again? (other than running at ender 3 speeds πŸ™‚ )
Ellis’ Print Tuning Guide
Ellis’ Print Tuning Guide
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