Apple wants to make sure that you use the official cable.

I got a Thunderbolt cable from a Mobile Iron conference, and was using it with my iPod Touch generation 5 for a while without problem even after upgrading to iOS 8.

However, that cable does NOT work with my new iPad Air 2.  Of course, official Thunderbolt cable works fine.  Seems like Apple did "something" again to make third party cable not working.  Sigh, that means now I need to carry the official cable around to make sure that I can connect without problem.  Not the end of the world but it's still irritating.