They refuse to accept the results that were obtained according to the Constitutional process.
The constitutional process allows them to do what they are doing.
Well, we do have a "living" Constitution, so it's ok.
Unfortunately, the Electoral College is a part of this constitutional process as well, and not all states compel Electors to be faithful, and only a handful of the ones that do have laws that can void the faithless elector’s vote before it it counted. Thankfully one of those states is Michigan.
So what they’re doing isn’t illegal, it is just very much a billion-to-1 hail mary. I want them to try as hard as they can, because their failure and the subsequent Trump inauguration will be extra-bitter for them.