This is not a coup. Let us be very clear, and very insistent about that. Very insistent. Not a coup. Definitely not. Don't be ridiculous.
Zimbabwe's armed forces have taken control of Harare and "secured" the presidential family at their residence. You will note that this sounds a lot like house arrest, but if I may refer you to the lines at the top of this post, it is of course nothing of the sort. We know this because of the statement given by General Constantino Chiwenga confirming that, despite superficially resembling a coup, what with the soldiers controlling the streets and the detention of the head of state, it is quite certainly not a coup.
The cause of the not-coup is the quite incredible oldness of Zimbabwe's long serving president Robert Mugabe. When, in the fairly near future the 93 year old sitting president dies, it is not clear who will succeed him (or win the resulting free and fair election by shattering landslide, in other words) but the two candidates left in recent years after the boiling down of the list were Grace Mugabe, the president's wife and popular jeer-recipient, and (now former) VP Emmerson Mnangagwa. The latter was removed from office suddenly last week, apparently with the direct involvement of the former. Chiwenga, an old friend and ally of Mnangagwa, then takes to the streets with his armed men and detains the president and his family for a vague reason which has not been elaborated on.
It's handy that he went on state television to assure the not-coupness of this action, because without that clarification it could be mistaken for one. By the untrained eye.