Couple of scenarios. Previously issued ET becomes invalid as it’s taking longer to clear the runway. With an ET you can hold below diversion fuel.
airspace congestion led to longer routing.
aircraft told of ET at diversion, or in organising diversion, that meant they would be landing with below reserves level of fuel.
Say the magic word, all stops are pulled, having been in a similar situation in the Middle East with an intransigent host nation trying to make a political point resulting in us having no diversion option, other than the country we were filed to land in.