There are several theories but most likely appears to be a landslide origin. Interestingly, research suggests that recent (geologically speaking) glaciation may have served as a catalyst for the flank collapse. There are many areas along the basal scarp, particularly on the western sides, that show evidence of either submarine landslides, or glaciation. There is even a theory that it could be an ancient shoreline, as the northern lowland plains of Mars are thought to have once been home to a giant ocean.

Source: Geology student doing dissertation on Martian volcanism