When unions gain too much power and the company can no longer respond effectively to market forces (particularly with hiring/firing), leading to the collective ruin they were talking about.
They wouldn't intentionally push it to fail, but they could easily push it very close to failing and then something else pushes it over the edge, happens time and time again.
Am I misinformed — that unions have actually been popular in the private sector over the past long while in order to trigger what you speak of recently?
Consider an actors union — actor unions have famously walked away from companies without much regard for the longevity of the company they walked away from on numerous occasions. They know in that line of work there is always another company looking to hire them, so there isn't a whole lot of incentive to care about individual companies.