A quick test:
Voidwalker's rank 6 suffering does 600 threat, according to
http://db.vanillagaming.org/?spell=17752. This means that it should require 780 threat to pull agro from range (130%).
I put voidwalker on passive, have it cast suffering once and back to passive before it melee hits.
I cast searing pain for 289, 271, 287, and crit for 546 with the mob still attacking the voidwalker. 1393 total damage without getting agro.
I hit one more time for 274 damage. This finally pulls agro.
My theory is that maybe searing pain is doing 50% threat, because 1393/2=697, which is below the 780 threshold, while another 274/2=136 put me above the threshold to gain agro.
Second test, again with the voidwalker on passive and using suffering rank 6 once, a single shadow bolt of 754 pulls agro immediately even though I am at range and this should be below the 780 threshold of pulling agro.
Maybe I'm missing something, but this seems weird.