I don't know, but I can make a wild guess. iOS 7 appears to have updated its Bluetooth support to add a few services, mainly related to low-energy operation. From what I can google up, it appears that to use the low energy mode, you have to use version 4.0 of the protocol, so clearly the iPad now supports Bluetooth 4.0. If you're using version 4.0, you must use the newer pairing protocol. At a guess, it was previously paired using Bluetooth 2.0 and the old pairing protocol (Bluetooth 3.0 was never widely adopted), but now that the iPad's Bluetooth stack has been updated, the iPad wants to use a newer version. To make this work, it needs to re-pair.