Dramatic drop in email open rates

Two weeks ago my team was seeing open rates in between 18-25%, depending on the communications we were sending. And those generally were our open rates of the last couple of years. Those open rates have now dropped into single digits. Has anyone experienced a drop in open rates such as this? We follow all standard email practices to optimize our chances of reaching inbox and avoiding spam. The only thing that changed in recent weeks was that we did a highly segmented set of emails one day to constituents in different states. So there were around 50 emails sent in one day, previously we sent 5 segmented emails a day max.