This is just basic email deliverability 101 stuff.
Gmail's algorithm has really tightened up this year. Open your spam folder right now and you'll probably see more than a few legit businesses in there.
Basically if your engagement (opens, clicks, unsubscribes, complaints, etc as a percentage of total sends) is poor enough, Gmail will dump you in spam by default. They don't just "trust" that you have permission to email the people you're emailing. So many businesses abuse this, and they're incentivized to keep your inbox clean so you keep visiting your inbox (and click those Gmail ads!).
If you're getting low/declining engagement over time the algo just assumes you have poor sending practices, which to be honest, if that's happening, you probably do.