Why have protests continued for weeks in Mozambique?

More than 150 people were killed in protests in Mozambique. Demonstrators claim a presidential election in October was rigged. The winner from the governing Frelimo party, Daniel Chapo, is calling for calm and a national dialogue. The runner-up, Venacio Mondlane of the Podemos party, is in exile and wants more protests. So what is driving this continuing unrest?