When laying people off, better companies will often accelerate vesting so that the departing employees get additional stock. For example, Google does this:

We’ll also offer a severance package starting at 16 weeks salary plus two weeks for every additional year at Google, and accelerate at least 16 weeks of GSU vesting.

https://blog.google/company-news/inside-google/message-ceo/j...

That's what the contract would typically say. But it's not uncommon to have accelerated vesting either when parting on good terms or with severance.