Why did Saint Francis of Assisi become the patron saint of animals?

Francis loved all of nature - plants, animals, the Sun, Moon and stars, wind, rain, etc.. Long before the term ecology became popular Francis understood better that anyone the wonderous way that all of creation was linked and interwoven. Read some of the hymns attributed to St. Francis for a better understanding: Canticle of the Sun, All Creatures of Our God and King, for example.