Okay, so whenever I delete message threads, it's a nightmare. As you slide left to reveal the delete button, they randomly start snapping back to the right upon touching the delete button. Only way to fix it is sliding to the left and waiting. When you wait the thread will snap back on its own but from there it subsides. You have to delete them pretty fast, one after another. I believe if you delete them super slow it doesn't start doing it, but who wants to take so long doing this? The bug exists on my iPad and my iPhone, I recently DFU restored my iPad as well too and the bug is still there. Probably among the most annoying bug in iOS 8 in my experience.