Email views abnormally low?

When I first started here, the company had monthly blacklist issues due to improper SPF records and using a host that was too small to handle the amount of emails being sent out by the marketing agency.

I went it and fixed everything with the SPF records and had them switch to a larger domain host and wr have been blacklist free for 4 months now.

At the beginning of the week our open rates dropped from around 50% to around 20% in a day. Yesterday, our views went back up to near 50%. Today, they are back below 20%. I've run several blacklist checks on our company IP and all of our separate IPs and everything comes back clean.

Any idea as to what could be causing such a large shift in open rates day to day?