What was the driving force behind free jazz being developed?

The first modern jazz style, which evolved in the early 1940s was intended primarily for listening rather than dancing and was developed in small groups.

Effectively, free jazz began with the small groups led in 1958-59 by alto saxophonist Ornette Coleman, from whose album Free Jazz (1960) the idiom received its name. Shortly afterward, saxophonists John Coltrane and Eric Dolphy and pianist Cecil Taylor began creating individual versions of free jazz.

Explanation:

It was called Free Jazz because without having to follow a predetermined structure, musicians were "free" to play whatever they wanted.