Mostly coincidence. There aren't that many data points. There are under a dozen Presidents in the last half century. Too small to attribute significance.
I do believe that there could be a small causative effect, but there is usually a very long delay between cause and effect.
Horseshit. We have 70 years of evidence to back it up, you cannot call it coincidence without another counter theory. Your opinion, while interesting, is irrelevant. It's clear as day in the data. You could say that the Ds govern with facts and science, the Rs govern with emotion. But what's far more likely is that the Rs believe in theories that run counter to economic principles.
GOP is the fun dad party and Dems are the mommy party.
Republicans run economy hot until it blows, then Dems get voted in to clean up. People get annoyed about taxes and regulation when economy is ok again, fun dad promises ice cream and pizza for dinner forever.. rinse & repeat.