The Bestiary text of Attack of Opportunity reads:
Trigger: A creature within the monster’s reach uses a manipulate action or a move action, makes a ranged attack, or leaves a square during a move action it’s using.
And this is the same for PCs. The text for Step reads:
You carefully move 5 feet. Unlike most types of movement, Stepping doesn’t trigger reactions, such as Attacks of Opportunity, that can be triggered by move actions or upon leaving or entering a square.
Cool. So the interaction between those is pretty clear, but there's a confusion in the wording there; the use of "or" entering or leaving a square implies that triggers that involve entering or leaving a square are not triggered by Step, regardless of whether or not they are triggered by move actions.
Now here's the text for forced movement:
Because you’re not acting to move, this doesn’t trigger reactions that are triggered by movement.
However, this doesn't make any statement regarding triggers that are caused by entering or leaving squares.
This means we have two categories:
- An action that is triggered by "movement" is not triggered by Stepping, nor by forced movement.
- An action that is triggered by "entering or leaving a square" is not triggered by Stepping, but it is by forced movement.
Which gives us problems in future and necessitates some very pedantic readings of Bestiary entries.
Ready and arbitrary triggers
This was in the playtest and I'm a bit sad it's not clarified here:
You prepare to use an action that will occur outside your turn. Choose a single action or free action you can use, and designate a trigger.
If the character gets to pick their own trigger, they presumably aren't obliged to include the ".. during a move action it's using" limitation on it. They could even specifically mention forced movement or Stepping in the trigger, and the interaction in that case isn't clear.
Summoning Rune, for instance, triggers when "a creature enters the cloud of magical sensors". Can this be avoided by Stepping into the cloud? (Presumably once in the cloud, you can Stride freely, because you are not entering it.)
Drowning Pit: triggers when "a creature walks onto the trapdoor". Does Stepping not trigger the trapdoor? Does forced movement constitute "walking"? Is there a distinction between walking and other forms of land based movement?
Quicksand: "walks onto the quicksand", again. Can you Step through quicksand?
Spinning blade pillar: "a creature steps on one of the trapped floor tiles". Even I'm not going to be pedantic enough to argue that this applies specifically to the Step action, but is it triggered by that or forced movement?
Lava Flume Tube: "a creature tries to leave the room". Does being forced out of the room constitute leaving the room without the creature "trying" to do so? Does Stepping count as "trying"?
Giant Moray Eel
This has the trigger:
When a target creature passes within 20' of the giant moray eel's hiding place..
Does "passing within" constitute "movement" and/or "entering or leaving a square"? Although interaction with Step in this case seems rather academic (since a moray eel would be hiding underwater and you can only Step with land speed), the implication that the eel could not strike someone who was pushed into their range seems a bit peculiar. However, if it is ruled that they can, then any creature can use the "passing within" wording within their Ready triggers and respond to forced movement.
This has the trigger:
.. moves into a space within the Glimmerling's Treacherous Aura.
Because this is a trigger that names movement, the implication is that Stepping into the Glimmerling's aura does not trigger the listed effect. It's not clear if this is the intent. Although the Gimmerling's aura is also difficult terrain which could not normally be Stepped into, there are abilities such as the Rogue's Light Step or a Cleric's Agile Feet, which would make this possible.
Monsters that don't trigger reactions
Many monsters, such as Zephyr hawk, have the following text:
This movement doesn't trigger reactions.
This presumably creates an additional, even stricter, category of movement which cannot even trigger Readied actions (since they are reactions), and the scope of not triggering is not clear.
If you are interested in this kind of exploitation, what you ideally want to do is to team up with or charm a Barbazu, because their Reposition ability allows a PC to be moved with the text "this movement doesn't trigger reactions". So a PC can move freely through a Glimmerling's aura, or a Moray Eel's hiding place, through a pool of quicksand, out of a lava tube, or past an entire pack of creatures with Readied actions, by having a Barbazu hit them with their glaive and Reposition them. This is pretty strange.