Engagement / Deliverability dropping with each campaign

I could use some insight on if I need to adjust something I am doing, or if this is a pattern that others have been seeing. I have been consistently hovering around a 40-45% open rate with a click rate of about 2% and an unsub rate of about 1% (150-250 or so each campaign). I have been experiencing a drop in open rates, click rates, even unsubscribes in the past month. A precipitous drop in fact. The past month the campaigns have dropped open rates from 45 down to 35 down to 20 down to 10%?! Unsubs are down to a couple dozen. Clicks are down but not significantly. I checked my deliverability with several spam/blacklist/firewall sites and all passed with flying colors, so it's not on our domain side of things. Has anyone else experienced a significant drop since the beginning of the year? I should note that I have been authenticated for a long time, the content didn't take a 180° turn, and my contact lists are clean and not new. Thanks in advance for your input.