Regarding Pox Bursters, and Pox Hounds (while at it), both check if their current target is doing a push action when within a certain distance, with no angle restriction (yes, you can push without looking at them at all, and they get pushed away from you). If so, a stagger is forced and guaranteed.
As noted, they only do this with their current target though, which is the problem.
When this isn’t the case, they have to be pushed physically like any other enemy, but they’re somewhat resistant to stagger, so sometimes you could push them a bunch and nothing might happen.
EDIT: that said, it’d be really nice if they also had some sort of check for players who aren’t being targeted. Sometimes the Burster can switch targets mid-jump and ignore the player that’s closer to them for the guaranteed stagger. And bots also just don’t have logic to account for this stuff so they end up eating Pox Bursters while also not allowing players to do the guaranteed stagger either..
I’m unsure if the 3 elites you mentioned have any sort of stagger resistance logic like that when damaged by someone who isn’t their target, I’d say it’s not the case as I don’t remember seeing something like that in the code, but I’d have to verify it first to be fully sure.
That said, I do know that Ragers and Scab Ragers doing their combo swings attacks gives them stagger resistance, and the same applies for overhead attacks on both Maulers and Crushers (combine this with havoc 40 and all of its modifiers to both player and enemies, and you might end up not having enough impact to stagger these enemies in the middle of an attack as reliably).
There’s also a mechanic that grants stagger resistance (or reduction, don’t remember which, but I know they’re different things) to enemies like Crushers, and possibly the other elites too, to make it harder and harder to cause a stagger on them.