The city sold the property, the former NYC*EMS headquarters, in 2014. They were on a temporary lease until the property owner needed it. Home Depot bought part of the Coca-Cola facility that surrounds the site in August for $70 million. I wonder if that had anything to do with it.
I know they've been looking for a large warehouse to keep these trucks in for quite a while, especially one that would be resistant to hurricane damage and flooding. Maybe they found one?
Here's an article from 2014, which failed because the Neighborhood Association did not want it in their neighborhood: http://sunnysidepost.com/op-ed-say-no-to-the-fire-departments-plans-to-use-43rd-street-location