If a Enderdragon is summoned in the overworld, most of the time a superflat world, it will fly inexplicably towards X=0 and Z=0.
MC-93964
Enderdragons in overworld are drawn to X=0 and Z=0
Resolved
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Jacob S. Reed
(Unassigned)
Unconfirmed
Minecraft 15w49b
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That seems to be intended as that's what it supposed to do in the End. You can run the following on repeating command blocks to stop it.
/entitydata @e[type=EnderDragon] {DragonPhase:0}
That should work. (Never tested it yet).