Wish I could say! I think Gmail relies more on Gmail users' own activity within the inbox. If it sees the majority of its users not interacting with your email, Gmail may move it to the spam folder. I'm not a deliverability expert (not by a long shot!) but that's usually my take on it when it comes to Gmail.
This has happened at Litmus a few years ago. To help us get back into Gmail's inbox, we segmented out our Gmail users and only emailed our most engaged subscribers (opened an email in the last 30 days) for 4 weeks. Once we saw an uptick in their engagement, increased the "last opened" to 90 days, for another 4 weeks. And so on. It was a long process, but it worked.
What percentage of your subscriber base are Gmail users? If you're noticing the drop in open rate was mostly Gmail users and they are a high proportion of your base, it could be that your email is landing in Gmail users' spam folder. That's what I instantly thought looking at everything else you've mentioned here.
At Litmus, we're in the process of getting this setup for our own emails. Working with our TechOps team to get it implemented. We'll share how we did it when it's ready.