Seems like a bug with the Volatile Familiar Explosion. It has been reported that they explode more than once. Reporting in Customer Service as a Bug. Attached is a screenshot. There are a couple of other screenshots in the Alliance War section. Apparently this does not happen to NPC characters but it does in PVP.
Sorcerer
Summoning
Volatile Familiar: Reduced the damage of this ability to 12% maximum magicka from 20% maximum magicka. This is a temporary solution until we completely resolve the issue where the Familiar is exploding twice.
I've been on the receiving end of this myself and I have to say I am surprised by the patch notes today. It's not about it exploding twice, yeah that is a part of the problem.... the problem seems to be that it explodes when you unsummon it. So Sorc who are (fill blank) just spam summon-unsummon till they turn blue and it explodes each time. Take away the explosion when you unsummon it and problem fixed.
I appreciate that work is being done on this bug as well as others within the game. It is disappointing to have an expectation of the bug being taken care of what appeared to be within a few days to a couple of weeks later and we really don't know how to do it, so partial fix. This does seem representative of some Quality Assurance issues in the game.
Hopefully added resources are applied in the Quality Assurance division to help make for a smoother, responsive game with minimal bugs and exploits.
The Road Ahead looks promising if the pot holes are taken care of efficiently.
This appears to still be an issue.... sorcs in cyrodiil are summoning/canceling their pets to aoe. Not sure how it compares magicka wise to things like Impulse, or if it's even worth fixing, but they obviously made changes because there is something not working the way it was intended.