The severance is only one month's pay, though, and they had to train their replacements for 3 months to get it. Not to mention that at the end of those 3 months the market will be flooded with all your ex-coworkers. Doesn't seem worth it to me.
The one time something vaguely like this happened to me, granted my boss made it very personal although in truth he'd lost his ability to sell, I would have been much, much better off telling him to FO and leaving on the spot, instead it triggered anxiety that eventually disabled me, which I later learned I had a genetic predisposition towards.
Money isn't the end all and be all, after all.
"Please your honor, it's such an easy temptation to fall into, when the articles are so infrequently more useful than the comments section."
More likely, if they'd left immediately, many of them could have gotten the equivalent of 4 months wages at their new job, since hopping is often the best way to get a raise.