It's most likely a set of users who expected something and did not have a good first user experience with your product. Maybe their expectation wasn't matched,

It's most likely a set of users who expected something and did not have a good first user experience with your product. Maybe their expectation wasn't matched, they were confused, or they were just browsing. But that's all conjecture, you need to find out from those 25%. Only they can explain their behavior. Learning about your customers should be top priority. I would definitely follow up with those 25% via a very friendly email, as mentioned in previous comments. Don't be surprised or upset if very few of those 25% reply to your email (would you - for a service you abandoned?). Good luck learning!