Why is the most common hydraulic fluid so poisonous?
Fume events are from two sources in the vast majority of cases:  Deicing fluid, and engine oil.
The Aviation Herald event linked was poisoning from (really severe) engine oil contamination (mainly the toxic TCP additive; it's the same stuff used as a lead scavenging agent for running 100LL fuel on low octane engines, and it's also the oil additive that Lycoming specifies for the H2AD piston engines for the cam spalling problem). Phosphate Ester hydraulic fluid had nothing to do with it.
The oil contamination comes from 3 sources:

A failed compressor section seal on the main engine at some forward bearing that is able to feed oil into the compressor upstream of the bleed port used for air conditioning.
A blown seal on an air cycle machine (the turbocharger-like cooling unit in an air conditioning pack) allowing ACM lube oil into the conditioned bleed flow.
An APU compressor seal failure (more or less the same as an engine oil seal failure) getting into the APU bleed and into the A/C packs.

Probably the majority of fume events are from oil traces, only enough to make smells, but which is really hard to remove, and deicing fluid (when airplanes get deiced and too much was sprayed into the engine and the engine didn't get enough of a purge run to clean the compressor out before turning on the bleeds).  Deicing fluid fume events happen a lot and are probably the most common cause of fume turn-backs.
Skydrol/Hyjet is mostly a hazard to the techs that work on the planes.  Anyone here who turns wrenches on transport airplanes will tell you that Phosphate Ester fluid is nasty, nasty stuff to get on your skin and probably the single most unpleasant aspect of the job.