The Early Church Fathers held that while speculation was OK, (and they speculated a lot) all doctrine could only be considered dogma, if it squared with Scripture. So the concept of Sola Scriptura was around and prevalent from the earliest days of Christianity.

Later, as the Catholic Church grew to prominence, they held that the Traditions of the Church, and "ex cathedra" pronouncements by the Pope were equivalent to Scripture. The Reformers, and Pre-reformers had a problem with this, and began using the term Sola Scriptura as a slogan to counter this trend in the Roman Catholic Church, even though the concept had been around from the earliest days of the Christian Church.

Tl;Dr: the concept has always been around, but the slogan gained prominence as a remonstrance against the Roman Catholic Church during the Reformation.