You could perhaps hire an improv coach for a team workshop. Level one improv is all about building team rapport and helping individuals feel more comfortable contributing in a supportive group.
Can't personally think of anything more likely to make an entire team quit in unison.
It isn't that the workshop was bad, or uncomfortable. They did a good job making it work for everyone. It was more of a realization of how blind the leadership was to truly understanding their people and finding ways to work together that made us all thrive.