Did it actually disintegrate that quickly? The Lancaster House Agreement was in 1979. While Zimbabwe has had pretty high inflation since its birth, it only started to have what would be considered hyperinflationary rates around the late 90s. And while the economy did fluctuate considerably during the 80s, it only properly disintegrated after Mugabe implemented the fast-track land reforms for which he is renowned after the year 2000. So we're talking about a period of at least 2 decades here.

That's not to say that Mugabe was a complete saint when he first became PM, on the contrary, he set out to destroy all opposition, including former close allies such as Joshua Nkomo who'd worked with him to overthrow the UDI government in the first place, almost as soon as he assumed power. But there were a whole load of other African countries that disintegrated much faster than even Zimbabwe.