What caused portugal to finally
withdraw from its african colonies?

Africa was being helped by USSR and Portugal was losing control
people were unhappy and with new found freedom in site and weapons they fought hard 
they decided to end their loses and withdraw 

The Portuguese controlled colonies in Africa for 4 centuries.  Many young men died in the Portuguese colonies trying  to keep the colonies during colonial wars. People were getting tired of that. Finally in 1974, the fascist regime was overthrown by a communist ruler who said from the beginning that he was for returning the land back to the Africans which he did in 1975