>>1813680
I use a T0 team so I can explain this.
Danu's field activates after a demon performs any action, but actions that trigger effects don't count until the chain is over or interrupted. The game considers that a single action, to put it in simpler terms.
For example, Sukuna Hikona's passive instantly triggers metatron's passive, so Danu's field doesn't trigger until the whole chain is over. But her field can trigger if the chain is interrupted by: an ally/enemy dying, a passive that's not part of the demon basic kit getting triggered or another demon triggering its effect earlier than anticipated (usually because of the team order).
A common example is metatron hitting a weak point and triggering his P2 effect, which is not part of Metatron's basic no panel kit, so the game considers it another action instead of a continuous one, therefore activating Danu's field.
That's why a fast Mozgus and Hachiman team can trigger by accident her field. Same goes for Hachiman and Marici or Mahakala and Mithras.