Pretty sure linked in saves your Gmail contacts once you login using Gmail. Similar thing happened to me too. LinkedIn is an abomination and so is it's ceo. Really surprised someone like Microsoft bought them.
They used to present something that looked like a gmail login, that if you used it, LinkedIn would take your credentials and spam all your contacts. I think they lost a lawsuit about that, and no longer have that dialogue... perhaps the app mines your contacts directly? This is one of the reasons I have never installed it.