I think it’s when they changed the stagger threshold ~5 months ago in 1.3? Some things/classes got hit worse than others. At a glance, it seems like a 50% increase and some new stagger_types.
In general I don’t mind all that much but as @MarxistDictator points out, the bullying class isn’t doing as much bullying. Ogryn got hit hard and bayonet pokes for instance don’t hit the crusher light stagger threshold.