Tax implications of working from another state due to coronavirus

You wouldn't be considered a resident of Michigan unless you move there permanently. For instance, buy a house or rent an apartment there (while selling/giving up your California one), change your voter & car registrations, mailing address, &c. Otherwise you're just visiting, even if the visit is extended.

FWIW, I've worked in California as a non-resident, for up to a year in Europe, and various spells while travelling around, none of which changed the fact that I'm a resident of the state where I own a house, vote, register vehicles, &c.

While qualifications might differ by state, it's fairly easy to find the particular ones that apply to you. (California can get picky because of its border with income tax-free Nevada.) You might also note that as long as you just work on line, and don't change any of those things that determine your actual legal residence, neither California nor Michigan knows where you really are. For all they know or care, you could be in Antarctica, aboard the ISS, or living in a yurt in Mongolia :-)

Probably the answer is yes according to the State of Michigan. As someone who travels for a work a lot (pre-COVID) and I have to file State income taxes in multiple states.

The main thing to know is that residency isn't usually the only factor in whether you owe income tax to a state. Often just working there (even on business trips) can incur a tax penalty.The rules vary by state.