Mages have ranged attacks that stagger you. They hurt pretty bad, especially when there's half a dozen mages stagger locking you for eternity.
^^^ edit: clarified by OP
Yes, but I believe those come from Spellbinders (the hooded ones), while he appears to have gotten first hit by a Mage's (non-hooded) elex mine and then Winter's Grasp while staggered. The WG part of it is what strikes me as unusual, unless it's simply a dragon projectile, though the same effect went off prior to the Leaping Shot as well, so they can't both be dragon projectiles. Watching it again, the Mage has an Ice Glyph at his feet, so perhaps there are unseen Spellbinders casting WG? Again, I thought only Despair Demons did that, so that's why I'm confused about the faction.
I think what's happening is the Barriered Vint Mage is casting Ice Mine, while another (offscreen) Vint Mage is casting Elec Mine and he gets hit by both back-to-back, and it's just Venatori faction.