Enemies can have a stagger count which relates to incoming stagger among other things. Normally this only increments on applying a stagger and resets on a stagger animation completing.
The Bug: Occasionally the stagger count can increase without causing a stagger animation. This is most noticeable on the Beast of Nurgle where weakspot hits can add large amounts of stagger count while never causing an actual stagger. Combined with a blessing such as No Respite, which gives extra damage per stagger count, this gives a ridiculous boost.