A bit of everything, 5 have been stopped due to cracks: https://en.m.wikipedia.org/wiki/Stress_corrosion_cracking

A few are having their once in a decade maintenance check

Some are waiting for maintenance which were delayed due to covid

We didn't train enough maintenance workers because everyone was talking about shifting out of nuclear in favor of renewable (we're using American contractors now afaik)

So mostly bad decisions and poor management, it's a political issue more than anything else, it worked fine since the 70s to more or less a decade ago