It's to block your console from updating. There's many ways of doing it like blocking URLs in your router, setting up a proxy, or the easiest method, using Tubehax DNS or the sort.
To set up DNS:
1. Open system settings.
2. Open internet settings.
3. Click Connect to the Internet.
4. Press X to go to Connections.
5. Click on your default connection.
6. Click Change Settings.
7. Click the right arrow and then click DNS.
8. When asked to Auto obtain DNS select Don't Auto obtain.
9. For Primary DNS enter:
104.236.072.203 (recommended) or, for Tubehax (which will not let you access YouTube) enter:
107.211.140.065.
10. For Secondary, either enter 0.0.0.0 (which will make the Internet fail if either of the DNSes go down) or enter in the DNS you didn't use for Primary.
11. Select confirm and test the connection.
12. Exit settings, and try to access the eShop. If it fails, you set up the DNS correctly. From now on, you should see a red X on Download Manager every time you boot your Wii U. Ignore the "System Update" in the manager, that means your blocking updates. You should also now see a prompt like "Some data failed to download" every time you press the Home button and access the Home Menu.
Note that you won't be able to access the eShop any more afterwards.