I haven't see that pop-up in a while. TBH, I don't know why it occurs but I do know it's a result of trying to download an app (or an update of an app) from the AppStore.
I think there are many reasons for this: (1) the AppStore is overwhelmed with all the new iOS updates, (2) Apple's servers are overloaded, (3) your iPad needs to finish indexing the iOS (if you've just updated the iOS.
The only solution I've seen that works is time. Let the iPad finish indexing (an upgrade isn't quite done when iTunes says it is, the software has to finish sorting and such). Or, wait for the AppStore/Apple's servers to catch up to this huge download.
I can tell you that I saw this pop-up when I first installed iOS 8.0 on my iPad (when it was first released). After a day, I've not seen it. So, all I've got is the suggestion to give it a bit of time.
Marilyn