After a summer of low COVID case numbers (in most of Michigan), we are now joining the rest of the country in significantly increased numbers. I’m sure there are lots of reasons, but from what I can tell here are the main drivers:

This virus is very contagious. Without full lockdowns and strict adherence to quarantine (and we’re not doing those things) it WILL spread, somewhat slowly at first, and then extremely rapidly, exponentially, until something happens to slow it back down (either a lockdown temporarily stalls the growth, or more definitively, we reach a certain level of community immunity) . This is the number one reason.

The weather is getting colder. SarsCoV2 - the virus that causes COVID-19, is from a family of viruses called Coronaviruses. These viruses are always worse in the winter time when the temps go down, partially because they last longer in lower temperatures, and partially because we all tend to congregate inside in the winter.
Schools? Not sure about that one. But, probably. Kids just don’t show the symptoms as much as adults do and so it is easily spread among them without them even knowing it. And then they bring it home to us.
Masking helps, but only somewhat. Its a bit like wearing a helmet so you can go out and play in traffic. Sure you’re safer with the helmet on, but ideally you would have stayed home in the first place. But it DOES help, so please keep wearing a mask when you’re out in public, particularly in enclosed spaces.
Finally, and this one is important - Its been almost a year since COVID became a thing. It was hard to lock everything down this spring and no one wants to do it again, even when they know we should. This pandemic “fatigue” is really getting in the way of being able to control it.