There's more to the story than the headline. Separation costs from his wife (a lawyer), plus child maintenance, plus burnout from lots of different roles. It's not just the mortgage payments.
If I understand high salary taxation correctly, there are no tax bands above £100K and you just pay 40% on the lot, which makes his take-home net £6K/month, of which a £2K mortgage is one third. I'm managing OK with my mortgage being one third of my net takehome. But I don't have an angry lawyer ex-wife and child maintenance to pay, or a stressful job.
Maybe mortgage increases were the last straw for him. But they are not the only reason for him quitting.