Really, now? Yeah, they can ask, but do data center operators just go "oh grid needs our power! Let's switch to backup!"
Of course they do. The grid is going to load shed regardless, better to disconnect in an orderly fashion (especially as the grid will pay you to do so) than wait for your local substation to disconnect you.
That seems unlikely. Why would a data center (or any large consumer of electrical power) be forced into a backup/contingency plan, when no crisis exists at all?
Because they get paid to do it. This is pretty basic demand side management that’s been the bread and butter of grid management for decades now. Grids will offer discounts to large loads that allow themselves to be disconnected at short notice, and will then pay those loads for the periods they’re disconnected. There’s usually multiple tiers of discount and payment depending on how fast a load can disconnect, and how often the grid expects to call upon that facility.