How does Siri come up with contact suggestions?
Relevant people

As Apple specified on its website page dedicated to iOS 9, with your Siri suggestions you can


  See the contacts you talk with most, those you talked to last, or even
  those you’re next scheduled to meet


I'd definitely suggest you to have a look to all that Siri suggestions include from here, in fact it doesn't just suggest you Contacts, but also


Places
Apps
News


Hope this helped you!