

The supports are only lightly connected to the object, to make them easier to remove. The force being exerted by the plastic as it cools is stronger than the light connection to the supports, especially over such a large area.
Probably there is a draft causing it to cool to rapidly. An enclosure, even just a cardboard box around the print area, would help.













OK, interesting, no draft. Is it PLA? Is the chamber being actively heated? What is the printer model?