Kargath Bladefist Aggro Reset

We were fighting Kargath Bladefist on Mythic lately and something really annyoing happened throughout the fights: He was randomly switching Targets during his Berserker Rush.

Now since you should kite kargath as long as possible during his Berserker Rush in mythic difficulty we also tried it. When a hunter or mage was targeted by the ability we also used Deterrence or Ice Block (Note that none of those two dropped the aggro). The main problem was that Kargath would sometimes reset his target and chop through the raid, onehitting everyone within seconds.

What is the cause of this aggro switch? I hear a lot of things in the forums. The most common ones are that it happens if someone (not the target) dies or that it's simply a bug.