That death is a hilariously unfortunately timed double hit from two enemies at once; Aetherial Behemoths are known for their significant Physical Damage output, with their charges doing quite a bit of damage to unsuspecting players!
You weren’t charged, but you did eat a 7.5k hit followed by a ~4k hit.
This coincides with the Behemoth’s autoattack damage (which is all it has for melee at close range, as otherwise it just fires some useless projectiles) for SR85.
Yes and no. It’s just the quirks of Physical Damage; Fabius is mostly Pierce (when one factors in Armor Piercing from his equipment…)/Cold which means you can negate 80% of his damage output with resistance alone (which is why he’s long been considered to be a not-so-dangerous foe). But against a large chunk of Physical Damage…
Let’s say you ate a max-hit from the Behemoth’s auto. That means you take 9382 Physical Damage, reduced by somewhere around 2200 from Armor. (on average, slot-specific hits will change that up a bit) The remainder is then reduced by resistance, which is much less than the 80%-to-everything that everyone walks around with; in your case, 31% which brings the Physical Damage down to 4900. Still significant! Plus there’s the chance of getting crit/resistances shred/mutators/whatever else, AND there’s still the Fire and Aether Damage left over.
Point being that while it’s perhaps strange from a vacuous standpoint, it’s very predictable.
There are champions with stackable rr (Wendigos iirc, also some of the Ugdenplants have -x phys res) and heal to full (the Korvan armors - I died lately to snap-healed Kuba in 163 - is it a new thing?) and other stuff that feels like wtf sometimes.
At certain level of SR those aetherial abominations (the ones that conceal themselves as crystals) are super dangerous with their Forcewave-like frenzy and keep you on your toes no worse than any nemesis.