I realized I used this term in a few descriptions without really describing what it is.
"Waggling" is what I call a pretty simple application of "strafing" - usually sidestepping one way or another though the direction is really entirely situation dependent - parallel to the way the boss is facing to see if he follows you when he goes to cast an ability. It's used for those abilities where a boss is going to turn, face, and follow his target to do a timed cast at the position where you are when the cast completes.
It's an especially useful little habit for melee to get used to because there's usually very little time for you to move if you wait for the other indicators - void circles and the like - as the "travel time" is frequently too short. For bosses like Erudax, the crash is already on it's way to the ground by the time the warning zone drops and you're not going to dodge it. As such, when the boss turns to face you a quick strafe one way or the other - I like to go quick right and then left to help confirm it's me, hence the "waggle" - can tell you if you're the target, because he will turn with you to follow, and let you know you're going to need to keep moving to dodge the attack.