The "scramble for Africa" was the European blitz to colonize and settle the African continent in the late ages of colonialism. There were many factors that were involved in this scramble.
First, while England, Spain, and Portugal had been the main colonial powers, industrialization and technology improved the wealth of other European nations and gave them the opportunity to begin colonizing. This is why Germany, France, and the Netherlands heavily entered the African colonies. Additionally, because of this industrialization, they believed that Africa would be a wealthy source of the materials they needed to keep up with production that would also be readily accessible (much more accessible than America or Australia).
This scramble was delayed, though, because the other countries were slow to develop to the point of being colonial powers, and, as Shillington relates, because England held a very strong stake to the free trade agreement, backing only those it thought would be beneficial to itself and supporting Spain and Portugal. Eventually, however, the other countries began to make headway and entered the continent. It was extremely successful in large part due to the vastness of the continent and the military strength of the existing colonial powers. No one wanted conflict, particularly with England or Portugal, so they divvied up the nations and laid claim to separate areas they could take without competition.