It looks like it might be a Sony Alpha mount, one of the later variants without the screw-drive autofocus.
Looks like an EF mount to me, but I have only seen the old Minolta A mount on lenses. It will be interesting if it is a Sony and so close to the Canon mount.
UPDATE: I purchased a different Sigma lens (18-200mm) but with the same contact layout (8 instead of 5) and it works perfectly on my 80D. So, not sure what's going on - maybe Sigma has some updated info from Canon??? Or the Sigma Alpha mount is a Canon clone??? I'm just happy the lens I purchased works!!
It seems to be contacts for teleconverters. https://en.wikipedia.org/wiki/Canon_EF_lens_mount see the "Communication protocol" section.
Tim, thanks for the link! I'm almost too embarrassed to admit that I actually have 2 of the lenses listed in the article as having the 'extra' contacts!