When you setup LinkedIn on Windows Phone through Settings > Email+Accounts, a connection to that social network gets added to your Microsoft account. Your account requests all of your friends/contacts from LinkedIn. Once it pulls them down, it syncs that information to the phone.
However LinkedIn allows users (your friends/contacts) to configure their privacy settings such that they do not allow their information to be shared with third-parties. This means that even though Microsoft Account is getting all of your LinkedIn friends/contacts, it will never be able to get all of them due those friends’ privacy settings on LinkedIn. So as it turns out, Microsoft Account sees the friends it cannot pull down from LinkedIn as “private private” (because the information is not shared by that person or persons) and it creates a contact entry on People hub. This then gets synced to the phone.
You can set your privacy settings here:https://www.linkedin.com/settings/data-sharing