It just happened en-masse, and has been happening to Johnson regularly this year. The ministers resigned because they didn't want to continue supporting Johnson and doing the things he wanted them to do. So how can you say it's rare?
"Everyone currently leaving Johnson's cabinet knew of his objectionable character before signing on, for example"
Ah, I see. Mind reading skills. Because you personally don't like Johnson, nobody who is now resigning and explicitly stating it is for honor reasons can possibly be telling the truth. That isn't a charitable or fair reading of what's happened.
"The modern Conservative party mostly has a reputation for corruption ... I teach political communication in a British university"
If that's meant to increase your credibility it has the opposite effect. An anti-conservative academic, what a shock. And one who teaches politics no less, whilst claiming (presumably to students) that none of the many ministers resigning for clearly communicated honor reasons are actually doing so, thanks to aforementioned mind reading skills.