I'm not alone in this, also happened to my friend, and also posted to Reddit:

https://www.reddit.com/r/techsupport/comments/1u7h672/browser_asking_me_to_select_certificate_is_it_a/

Seems like somebody definitely misconfigured something somewhere. The question is then was this the direct result of the misconfiguration, or someone abusing a misconfiguration. Client certificates can be used to identify people so this is a good way to mass collect certs. This data could also be used to analyse client certificates that are around.