Randomly - the reporting on this is SHIT, but from what I can gather, it sounds like they were running a trial on extending the screening age at the top (and bottom) end of the 50-70 age range across 65 screening sites. Women were picked randomly to participate, and half of those picked were screened, half weren't. The problem is that for the 450,000 who weren't, the computer cancelled the last screening letter.
Much as I loathe Jeremy Hunt, this isn't his fault. It's the national office's fault, a body that's now part of PHE. They run the computer system. They are the ones that should have been crosschecking and ensuring there was a failsafe built in, not GPs.