Mounds and pyramids are ubiquitous throughout the world. Worshipping sky-gods and celestial events is one supposed reason, but mounds have other purposes as well. A mound is easily defended, as anyone who ever played king of the hill knows. A mound is a great thing to have in a floodplain. A mound that is built by hand labor is a sign of wealth, power, and success. A mound makes a great place to look out and make sure everything is going right.
Ideas have been shown to travel where the culture cannot. I can easily imagine a couple of wandering or exploring people from one culture being captured and saying something like "Well, at home we have these huge two mounds dedicated to the fertility goddess. You guys are just losers." That would start a flurry of mound building.