From memory (and one of the pilots can correct / enhance my answer as they see fit)

It's not an unairworthy aircraft or someone that would keep the plane grounded, it's likely to me a minor technical issue that means the plane is (temporarily) loses its ETOPS certification. ETOPS certification being needed to take a more direct routing across to the Americas & Canada etc.

It could be something as minor as one of the air conditioning packs not working, the APU is inoperative etc.

More about ETOPS here (Yes I know it's Wikipedia)