They likely had some outlandish request or policy that was anathema to the department’s mission, and just assumed that they would cave. Seems like good leadership stuck to their principles, and the good leaders were followed by teams who weren’t willing to lose that.
If the owner is smart, they’ll backtrack, make concessions when hiring everyone back, and learn from their fuckup. In reality, I hope the new company that forms from the exodus finds fast success.