What was the design reason behind the Rathalos Armor Set?

[Berethos]

I'd assume it's because that coupled with the existing things that got XIV its T rating would've pushed it over the edge?

I'd say that's likely.

MHW is T for Blood, Mild Language, Use of Alcohol, and Violence.

FFXIV is T for Alcohol Reference, Animated Blood, Language (so that right there bumps it up a bit on the scale in comparison), Suggestive Themes (another one that tends to cause rating boards to bump higher), and Violence.

Add in a dismemberment of a body part where you can actually see the bone, muscle, connective tissue, etc., would probably bump up the Animated Blood to Blood and Gore (which is for the depiction of the mutilation of body parts), which could be enough to take it from T to M on top of everything else.

Assuming it wasn't a joke that got lost in translation.