the word "secular" is used to describe something that is not based on or connected to religion. why is averroes considered the "founding father of secular thought"?

This basically because he argued for the greater plurality of the world views

He was a multidisciplinary Spanish-Arabian Philosopher, Lawyer, and Mathematician, who lived between 1126-1128.

His teachings movement averroism gained prominence after his death. And this influenced scholasticism in Western Europe in later ages.

He defended the Greek philosophy against the Ash'arite theologians al-Ghazali. This made his teachings to be controversial to Islamic faith, but due to significance of the teaching to western European thought, he was regarded as the founding father of secular thought. Regarded as the commentator in the by the Christian of the Western Europe.