When in a dimension other than the End, such as the Overworld, an Enderdragon when spawned will stutter as if paused on a nonexistant end portal from the End. Then after a few moments, it moves again for a short distance and pauses again.
To test, summon Ender Dragon in the overworld and observe.
Fixed in 15w32a, but dragon behaves like in the end: MC-85304