How does that not fit a chain of imagined causality? Isn't that what the "why" is addressing?
Because in the game in question, Risks determine who decides the outcome of the triggering question. The why, if not previously defined, is subject to changes by other participants. When something causes the GM to think, "That has interesting potential to go wrong in interesting ways" or when something narrated has strong potential to do mechanical state changes, the Risk process is triggered.
The Risk process involves everyone in scene and willing to be (or defined to be within the fiction state) involved or who has a significant setting state or fiction state stake in the affected character, gets to develop a dice pool of d6's, set some aside for making additional statements (those dice are "wagers"), and roll the remaining non-wagered dice for determining who speaks when about the Risk. The players each state their intended reaction to justify the dice in their pool. Note: At least 2d must be rolled dice; if you can't get 2d, you are excluded from the risk.
Anyone who's rolled dice total is less than 10 loses any wagers.
Of those left, the highest roll discards one wager, and decides the core triggering question(s); in my exemplar, both success of the attack and the reason for the attack. all other players lose half their wager dice. Then, in descending roll order, each participant with remaining wagers gives a "Yes [and/but]..." about anything in the same scope as the Risk. Anyone who rolled is automatically in scope.
By introducing the NPC unnamed and defining only the what and how,
everything else about that character is fair game. GM perogatives in B&H are different from both Traditional and AWE/PBTA GMs... The GM perogatives are to introduce NPCs without having to run them as a risk; to decide something has room enough to be interesting and thus trigger a risk, and to award honor points.
Anyone paying attention to the game is, per the rules, allowed to introduce new characters, but doing so triggers a Risk to define the character,
unless the character is introduced by the GM or
the character is introduced by doing something that needs immediate resolution (such as an attack or something making a major setting state or fiction state change). In the first case, any spots on the sheet not generated before introduction can only be filled in by Risks; in the second, the action is resolved without a definition of the acting character (including motivations), and so all others with stake in the scene generate pools based upon how they intend to react to the trigger, but the character doing the action is not rolled for.
This causes NPC's introduced by action to be the trigger for a whole lot of related risks - the introductory action, then, if no one filled in the blanks, a second risk to determine their stats and motivations.
The exemplar has only the following fiction state truths:
Someone (no statement of whom) not already in the scene enters
That unnamed and undefined someone attacks the Daimyō
There's no GM secret (it wasn't the GM introducing it - I was the GM)
Since only the GM gets to introduce prefilled character sheets into play, the player would have to use wagers to define anything past the presence of the katana. One wager per attribute, aspect, reputation,
giri (role or duty), piece of gear, or non-mechanical element (clan, family, name, age, gender/sexuality, appearance). NPCs can sometimes be used a lot before all of the sheet is filled in.
So, literally, the NPC has no motivation defined - neither as GM secret, nor as player secret, nor as mechanic, only that they exist and attacked the Daimyō. The introducing player is literally making the risk about the why; whether the Daimyō is killed or not is almost incidental. Everyone who rolled 10+ gets at least one thing to say about the situation (using "Yes, [and/but]..." statements, one wager per each).
Its the ultimate in quantum NPCs... the introduction was a non-sequiteur - no defined reasons given, no identity of the assassin, no reasons exist until after the determination of success.... and no prior presence unless and until someone decides to use a previously defined NPC and spends a wager to make them the assassin..
In the session in question, DB rolled a 9 on his rolled 4d6... with 5 wagers set aside, and so whether or not he had an intended person became utterly irrelevant when one of the players still in defined it was a miss, another player defined who it was and a third other the why.
If DB'd instead summoned up the character without declaring an immediate attack, it would still have been a Risk, but the resulting actions would have been subject to the character being defined away from any intended purpose.
Should the DB have narrated, "Rising through a floor hatch, Ichirō screams 'Death to my usurping little brother,' while attacking him" there's an identity and an action, and a fuzzy but defined reason.