Has anyone had emails flagged as spam by accident? What was the issue and how did you fix it?

So we just had a situation that's making me question everything I thought I knew about our email setup and I'm hoping someone here has dealt with this. We sent out a routine nurture campaign yesterday to about 10K subscribers. Same template, same segment, same sending frequency we've been doing for months. Normally we see 25-30% open rates on this series. This time? Campaign landed in spam for what looks like a huge chunk of our list. I've been digging into it all day and I'm stumped. We haven't changed anything obvious. Domain reputation looks fine. SPF/DKIM are set up correctly, I think. Content isn't wildly different from previous sends. No new links or anything that should trigger filters. But something clearly triggered it. Has this happened to you and what helped? Honestly, we need better visibility into this before campaigns go out. Flying blind until something goes wrong isn't something we want to keep happening, especially as we scale. Any tips appreciated.