so, like Tufts?There were a number of issues.
#1 they weren't just dominant, they were jerks about it. They were perceived to be running up the score in football games unnecessarily, and when grumbling started, reacted by beating St. Olaf 97-0. It also wasn't necessary- St. Thomas had the autobid wrapped up. And St. Olaf was 3-5 in conference, so it wasn't even an 0-8 disaster of an opponent.
#2 the size and scope of the school had changed. St. Thomas now has over 6,200 students. The only other school over 3,000 is a women's college (St. Catharine) at 3,100. St. Thomas also was perceived to be admitting athletes that the other schools couldn't or wouldn't get in.
So, the other schools in the conference (likely excepting St. John's) all decided to leave and form their own conference unless St. Thomas agreed to leave. Hence, they left "unvoluntarily" but still left.
Basically, it's like the rest of the NESCAC woke up one day and realized that Middlebury had slowly doubled enrollment and started ignoring academic standards for its athletes and hadn't lost in a decade.