Overseers have nothing to do with that. As Annatar and InfiniteVoid have indicated, this is the effect of the Charged Missiles upgrade. Both techs are at tier 4 military so its likely you researched both at approximately the same time, which is why it seemed to you that they were connected.
Charged Missiles increases the range of phase missiles when activated. it also causes a portion of the missile's damage to shoot out a pulse beam and hit adjacent targets, effectively giving the missiles AoE effect.
