There will always be one that drops right before/during Defensive Systems cast. It can drop on the Stormcaller tank (player, not boss) and whoever is at ranged. I've never seen it drop on the melee/healer group. If you're having issues with double destruction right before defensive systems and your soakers are getting gibbed by the shield, you can try to push the dps to only get one double destruction between defensive systems. If you can't manage that, you can either wait to push defensive systems (not something I would do because of enrage, if you really can't push DS faster). Or just have your soakers pay really close attention to where they are and avoid any excess damage since the phase is extra hard on healers with DD up on 2 targets.

Good luck.