I have always played with that the charger strikes first. Or if someone has special abilities that allows for him to always strike first, such as the mark of slaanesh.
It kinda makes more sence if the charger strikes first if you do not have such an ability.
An enemy unit comes roaring at you, you spot the character within it, you step aside and yell challenge towards him, he leaps of and starts running towards you.
He slams right into you so I would say he strikes first.
Unless he slams right into you and because you have the mark of slaanesh you strike him before he has time