I have to agree. This is by far one of the most confusing mechanic i have ever seen. There are occassions you get for the entire fight nothing and then in the last 10% or so he suddenly gains 4 to 5 stacks and noone has an idea how that happend even when you do everything right.
i heard also many theories. the interupt & cooldowns are false for sure. I believed the 'don't break the root' was the most accurate reason at first.
But a recent kill has now start to question everything again about this stand & fight mechanic.
Considering how confusing it is, its ether bugged or simply not enough information is provided to the player on how to deal with it.