Plausible for some, not others. I've seen LinkedIn suggest I connect to "people" that aren't people at all, but role addresses. E.g., reception@[my oral surgeon]. Others are email addresses that are ancient, 15 years old at least, or for domains that no longer exist.
No idea how they've gotten access to that, but I've never knowingly authorized a connection to my address books.
This is the behavior I have observed too. There are addresses that could not plausibly be gotten by any means other than unauthorized scraping of my mail or contacts.
The only other mechanism I can think of is that maybe they're getting them via Android or iPhone apps (not necessarily linkedin's app) with less-than-upfront disclosure of what the app's doing. Either way, it's slimy behavior.
That person may be in your address book, which you've never shared with LinkedIn, but you are in their address book, which they clearly have shared.