How to disable Wifi Password Sharing
You may have enabled iCloud Keychain on your devices. If enabled, WiFi networks and the passwords associated with them are shared:

What information does iCloud Keychain store?


iCloud Keychain stores credit card numbers and expiration dates—without storing or autofilling the security code—and passwords and usernames, Wi-Fi passwords, Internet accounts, and more. Developers can also update their apps to use keychain, if the app is on a device that uses iOS 7.0.3 or later, or OS X Mavericks 10.9 and later. If you don't see items stored in iCloud Keychain, learn what to do.

From: https://support.apple.com/en-us/HT204085