Oh my heavens, I would have been physically sick finding that.
My guess is the UTH got too hot and caused the glass to expand, cracking it. It shouldn't happen under normal conditions, but if the pad overheated or there was a defect in the glass this is the outcome.
Obviously, if you ever go to move the tank in the future it would be risky. That kind of crack is seriously destabilizing against twisting motions.
It may end up being okay where it is if you don't mess with it much. I would recommend watching the surface temperature of the glass and making sure it doesn't fluctuate too much too fast.
You can help to shore up the crack and prevent accidental cuts by covering it with aquarium silicone. It won't be pretty but it will be functional. The crabs cannot be exposed to the fumes though and it may take a couple of days to air out.
I have used packing tape on cracks on smaller tanks as a lamination technique to secure the glass. That one is instant gratification but I am not sure how well it would work on thick glass.
