A few points from the article I think are important:

[The military] said they were annulling the results of Saturday's election, in which President Ali Bongo was declared the winner.

Military officials later said that Mr Bongo had been placed under house arrest and one of his sons arrested for treason.

His overthrow would end his family's 53-year hold on power in Gabon.

[The military announced they were] cancelling the results of the election and dissolving "all the institutions of the republic".

[The opposition argued the election] was fraudulent.

This is the eighth coup in former French colonies in Africa in the past three years.

Does anyone have an idea why these coups keep happening? I am not familiar with the politics of many of these nations.