I've only recently started playing a magicka sorc for the first time this week, but the execute and shards makes this happen all the time. I thought it was just something sorcs must have to put up with.
The odd thing is I'm using both morphs of those skills that are rare, the AoE versions.
It happens when you don't get the golden ghost if you miss 1 he will get stronger and you can't kill until the effect ends.If you miss 3 he will put the soul churn on your ace.