What else could possibly motivate a prominent Michigan democratic politician to make such an unhelpful intervention so late in a tight election?

Probably the same motivation that led Uncommitted to so pointedly withhold their endorsement of Harris in the general election, absent some public movement - in rhetoric at a minimum - on an issue they felt was important.

I'm not in Michigan, obviously - but from the news reporting, it appeared that there were plans for a joint appearance. That's unlikely to happen without the El-Sayed campaign's blessing, so clearly his campaign felt that her participation/endorsement would help in the final days. She levered it into an opportunity to try to get El-Sayed to move on an issue she wanted him to move on, else pay the price of an obvious and public rift.

It seems like terrible politics to me - but I thought the same of what Uncommitted was doing back in the day. But, if you're looking to understand what happened, I think that's it.