Just as I asked myself that question this evening, I saw the following video from WOOD-TV on my YouTube subscriptions page, which answered my question.
It's a local issue. Michigan is subject to supply disruptions from as far away as Texas, including refinery shutdowns, and there were two of them recently. Fortunately, both will soon be over, and gas prices will drop. As I'm fond of saying, that's good news, and it's not a suppository.