Weird, I did several elite runs today as mage and did not notice anything similar. Neither on my time shift's clock nor any other mage's. Sometimes the clock is delayed due to lag and doesn't register as charged skill even though you did charge it because it happened that the moment you released it, your ping was on a spike. So that would explain why they fail to root (since you need charge for that upgrade to be set off).

And yeah, you guessed it, this mainly happens on large pulls when the screen is filled with a ton of mobs. No better suspect for such a map than WT4.