There are several reasons an email can report as opened instantly.
- McAfee security has been known in the past to auto-open every email to scan for spam content before it is delivered.
- Fake email addresses will have an auto-open setting as well. It is a way to farm for web pages and email addresses that they can then sell.
You can look at your immediate opens and check for repeat email domains- like everyone from a company or a university. Then look at the opens and do a gut check. Are the addresses clearly spam emails?
With some after-send filtering and you can get your reports back to normal again.
Note: also look for autoclicks on the “view as a web page” link for spam email addresses. Cleaning up your list is good for all involved.