Well, the main thing was that the guy in the middle of things was already involved in another duel (though was thusfar undamaged). So four mechs were involved in two separate duels, and one of those four got between the other two while on the way to a fourth.
Rulewise, I'd just use the dezgra point system. Under the system (First published in
First Strike! as I recall) a unit that intentionally moves out of line of sight earns a dezgra 'point'. There are other ways of earning points, such as moving out of range for all its weapons. A unit can earn 1 dezgra point per turn from what I remember, and if it hits 3 points, his opponent can declare him dezgra and Zellbrigen is dropped against him.
If you got a turn without earning any dezgra points, you can lose a point.
It was originally created to prevent a spider challengine a Direwolf and kiting it around the map while other units beat on it- with the Dire Wolf being unable to fight back due to adhering to 'Level 1' Zellbrigen.
In this case, I would say if a unit has already moved- and then an opponent's mech moves so as to block its LOS,
it earns a dezgra point. If the opponent moves in such a way as to hide behind another unit (either faction) and breaks LOS, it earns a dezgra per the original rule.