By default Windows 10 improves download speed of Windows Updates to all users by sharing Windows Updates in a Peer-2-Peer fashion. Once the update is downloaded to your PC (from the Microsoft site and other Windows 10 users), it is then uploaded to other Windows 10 users.
Obviously this reduces your bandwidth and may cause a security concern. To disable Peer-2-Peer update sharing on Windows 10:
Go to the Charms bar
Choose how updates are delivered
Change the option to PCs on my local network
I imagine in a future update to Windows 10, this default will be changed to disable P2P updates by default. For now, though, you’ll have to disable P2P updates manually after upgrading to Windows 10.