The change happened after a coup, headed by the legendary Marxist and pan-Africanist Thomas Sankara. These ideologies are also reflected in the flag, with a Marxist-inspired design and the pan-African color palette. This makes the flag not a nice example of a simple yet beautiful design, but also a good example of how vexillology can be used to express an ideological stance.

The flag of Upper Volta had a quite different feeling, rather leading the thoughts to, well, the German Empire, which incidentally had an identical flag:

Flag Upper Volta

Wikipedia page about the flag