Why does this happen to my ipad icons??

Tap on them. Once they open in safari the thumbnail logos will appear again. However, since they are shortcuts if the website owner hasn't added an icon to be used when shortcuts are made you may still see the white squares.

I also had this issue and decided to look further into it. I found that changing the ipad language and then changing it back gave me back all my icons. This tip was found in the following discussion post :Lost bookmarked to homepage icons ios 8