Zimbabwe was the bread-basket of Africa until Mugabe, for political reasons, decided to reallocate the land of white farmers. Instead of reallocating the land to the black labourers who worked it, he used it to reward political loyalty.

That is the reason that Zimbabwe is collapsing: Those who own the farm land now have no idea how to manage it.

This needs to be better publicised, to persuade black people, inside Africa and outside, that Europe and the US aren't always acting selfishly, but do sometimes have good advice to give. When the US and Europe criticised Mugabe, South Africa backed him against that criticism - citing the history of slavery and racism.

China is now moving into Africa, supporting people like Mugabe, who Europe and the US won't touch. China doesn't do anything out of the kindness of its own heart, always looking for benefits. Of course, Europe and the US normally do this as well, yet China is being accepted with open arms by many African leaders.

Because of all the politics and history behind this, the well-intentioned advice of Europe and the US is scorned. It is gross stupidity - wilful ignorance fueled by grudges that, not matter how richly supported by history, are nevertheless destructive.

Wouldn't surprise me if there were multiple factors playing into this famine.  Africa as a continent is a mess.  As Africa fails, the rest of the world shall suffer like dominoes.  

Zimbabwe is bordered by South Africa, Botswana, Zambia and Mozambique.  
From wikipedia:

The government of Zimbabwe faces a wide variety of difficult economic problems after having abandoned earlier efforts in developing a market-oriented economy. Current problems include a shortage of foreign exchange, soaring inflation, and supply shortages. Its 1998-2002 involvement in the war in the Democratic Republic of the Congo, for example, drained hundreds of millions of dollars from the economy.

According to official figures, inflation rose from an annual rate of 32% in 1998 to a high of 1204.6% in August 2006, a state of hyperinflation. The exchange rate fell from 24 Zimbabwean dollars per US dollar to 250(000) Zimbabwean dollars per US Dollar (official rate) and 850(000) Zimbabwean dollars per US Dollar (parallel rate), in the same period.

Let's not forget in the midst of the collapsing economy and inflation, there is a high AIDS rate that is crippling Zimbabwe as well.  Wars can destroy some countries for decades, making them incapable of dealing with disasters like epidemics or weather problems.  The famine is a huge problem in Zimbabwe, and we can place a good deal of blame on Mugabe, but there are so many different factors playing into the problems of Zimbabwe, it would be difficult to comprise a solution other than treat each of the problems as a symptom of the overall illness to that country.