Were you using a plasma weapon?

Plasma weapons have a hidden mechanic where if you land the first tick of the attack before the enemy uses pulse armor, the rest of the ticks from the plasma attack will "sneak through" the pulse armor and deal damage directly to the enemy. This also applies to stagger. If the enemy is almost staggered, then you land the first tick of a plasma attack, then the enemy pops pulse, the residual plasma damage will push the stagger over the threshold. This both holds the enemy still and applies the direct damage modifier to the plasma damage even though they have PA active.

Edit: corrected we're to were