Mugabe and Tsvangirai are currently governing Zimbabwe together (with Tsvangirai as Prime Minister) through a power-sharing agreement.
Tsvangirai is specifically angry that Roy Bennett - who is white and is Tsvangirai's nominee for deputy agriculture minister - was subjected to allegations of terrorism.
Dugger says that Tsvangirai is, more generally, motivated in part by a desire to "wake-up" regional monitors and in part by discontent within his party:
"Mr. Tsvangirai’s strategy appears to be in part an effort to get senior political leaders in the African Union and the Southern African Development Community — guarantors of the power-sharing deal — to put pressure on Mr. Mugabe to act in a more conciliatory manner.
But the move also reflects rising anger in the ranks of the Movement for Democratic Change. Mr. Tsvangirai, who outpolled Mr. Mugabe in elections last year but withdrew from the runoff after an onslaught of attacks on his supporters, has sought to put a good face on the deal in recent months. He had argued that the country and its devastated economy had stabilized."