Sudden terminations without cause are legal.

Mass terminations without timely notice, or pay in lieu of notice, are not. Disguising a mass termination as a collection of "performance" based individual terminations is a method that many companies have tried and failed, because...get this...WARN regulations in the US and EU don't care about the reasons for termination unless it's a "for cause" termination and you absolutely need to document the cause for those.