I've seen mulberry leaves die back to a mild frost, so if it was windy and 30's that is a possibility. You say they were bareroot, but after potting leaves fell off. I'm struggling with the timeline as bareroot
trees should arrive before leafing out. If they were fully leafed out as bareroot plants, it would make sense that it would drop them all, as it wouldn't be able to keep up with the demands for very long after leafing out without an established
root system in the soil. Mulberries are very resilient and I would imagine once it gets its feet settled it will send a sprout from lower on the tree somewhere.