That really is very odd behavior. I don't see why s2Member would be connecting to wp-login.php like that. Was there any other clue in the logs, about the activity?
What happened a couple days ago? Did you update or change something in your site?
Have you customized s2Member in any way? Do you have must-use plugins added? or 3rd party add-ons?
You could try renaming the current s2 dir to something else, and upload a clean copy. See if that makes any difference.
Another thing to try, would be deactivating the other plugins instead, add s2Member first, see if that odd behavior repeats, and if not, add the other plugins one by one, checking after each.
If some plugin is conflicting with s2Member, then even if you add s2Member last and then you get the problem, doesn't mean it's coming from s2. If there is a conflicting plugin, it wouldn't show the problem until s2 is back there.
Anyway, I'm just trying to guess possibilities so you can troubleshoot, so we find what is really happening.
I look forward to your update!