I think it requires a def file with constraint data? I'm not sure how else we'd go about that so lemme do ya a look here. Aight, so digging into some of our def files I think I can point to the block that will properly handle your ragdoll. These are used in TDM but considering these look pretty standard, I'd say they're inherited functions from the original D3 ragdoll system.
One thing to take into account, is if you're indeed using a brand new monster (and importantly with new rig with anims) you'll most certainly need to define this somewhere for the game to understand your rig's ragdoll constraints. I wanted to create a roller bot at some point but I haven't delved enough into this process myself to be %100 certain but hopefully this will help.
*Taken from tdm_ai_base.def*
"editor_float mass" "Character's mass in kg."
"editor_var ragdoll" "Specifies the .af file to use for a ragdoll when the creature dies"
"editor_var ragdoll_slomoStart" "Start time of ragdoll slow motion relative to death time, defaults to -0.2"
"editor_var ragdoll_slomoEnd" "End time of ragdoll slow motion relative to death time, defaults to 1.0"
"editor_var ragdoll_jointFrictionDent" "Joint friction dent on ragdoll."
"editor_var ragdoll_jointFrictionStart" "Start time for joint friction dent"
"editor_var ragdoll_jointFrictionEnd" "End time for joint friction dent"
"editor_var ragdoll_contactFrictionDent" "Contact friction dent on ragdoll."
"editor_var ragdoll_contactFrictionStart" "Start time for contact friction dent"
"editor_var ragdoll_contactFrictionEnd" "End time for contact friction dent"
"editor_bool af_push_moveables" "whether the monster's articulated figure pushes obstacles while they're alive."
If it's any help, you may want to compare this to whatever default base you're pulling from in stock D3 assets. "Editor_var_ragdoll" looks to be of the greatest interest to what you're encountering, because I'd imagine if the game cannot locate a proper .af file; it's likely to be kicking it to Tpose for lack of identifying the correct information. If you do have all this information lined up in a working def file, I'd second check the lines for typos as well as the integrity of the af file itself.
EDIT: Did a tad bit more searching and I think this .af file crops up in one other spot (directly on a character def file). It's worth assuming the articulatedFigure declaration is searching the .af file so maybe this is closer to what you're needing.
* ============================= CORPSE ============================
"editor_usage" "Ragdoll for builder_acolyte"
I'm starting to get kinda curious how to rig up some of this stuff myself now that I've been rummaging around in these folders a good bit. Either way hope that helps man!