1.On selecting target, charges that location, that target is currently at
2.Selects target, short delay, then charges, the new current location.
3.Charges selected target, where ever the targeted player is at Charge cast.
He charges where the target is when he finishes doing his wind-up animation - slamming the ground with his tail and nodding a lot. If the target runs toward the boss and gets in melee range before it goes off, the boss will barely move.
And while it wasn't part of the original question, in our experience he also seemed to prioritise the person who was furthest away from him (while still being within his 100 yard charge range).
We had some success with deliberately baiting him to charge one of the ranged/healers, so we could keep him well away from the door we were dealing with, which gave us a hell of a lot more room to move with the adds & made it much, much smoother. That extra room to move was what got us the kill really.
Boss "casts" charge at player and then charges said player wherever he is and stops at his/her location, then starts the double swipe. So aslong as you are out of the way fo the raid, you can simply stay where you are and move to the side once he charges you.