What causes certain bosses to lose poise: to collapse for a brief moment allowing for critical hits / riposte?

When fighting various bosses I've noticed that by damaging(?) certain bosses can be poise-broken: they collapse for a brief moment allowing for a critical hit (like a riposte without parrying).

It's unclear to me why exactly this happens. First I thought it was related to the weapon weight, as it only triggered with Ultra Greatswords, but later I also managed to trigger it with shorter/faster weapons. Is it perhaps related to doing a certain amount of damage within a certain amount of time?

The bosses on which I'm clearly able to break their poise without a parry:

• Nameless King (and his bird-dragon)
• Twin Princes
• Darkeater Midir

Solution 1:

Poise works in a way similar to health; specifically "poise health". Everything has poise health, even you, the player. Certain armour and spells can boost this, and every attack, and some spells can damage this.

Over time, poise damage will build up, to the point where the counter reaches "0", and the target is staggered, allowing for a riposte. However, over time, if you do not attack them, eventually this "poise health" will automatically recover, meaning you will need to deal that damage all over again in order to stagger them.

Basically, the bigger the weapon, the more poise damage you do.

Source