Are you using "reason" as synonymous with "cause". Because you are replying to a post which is one in a series of posts by me that talks about causation.PC fails a save against a suggestion spell where the suggestion is to say, "Help! Help! The elves are killing me!" The PC's player then says, "Help! Help! The elves are killing me!"
How is that not a reason from the imaginary world why a person at the table says X rather than Y?
But in any event, the example you give can be fairly straightforwardly analysed. I will refer to two game participants, Person A and Person B, each of whom (in the example) is controlling one character. Person A's character I will call M and Person B's character I will call N.
*Person B says "N casts suggestion on M - Say, 'Help! Help! The Elves are killing me!'"
*People at the table - including Persons A and B - update their shared fiction: it now includes N casting a suggestion spell on M, and it further includes N saying to M, a part of the casting, Say, "Help! Help! The Elves are killing me!"
*People at the table consult the rules - either literally, or via recollection - and note that, in order to work out what happens next - is M ensorcelled? - it is necessary for Person A to roll a saving throw. [We could state the rule as follows: If, in the fiction, one character casts a suggestion spell on another character, then a saving throw must be made by the controller of that second character to determine whether the fiction becomes one in which they are ensorcelled by the spell, or remains one in which they are not.]
*Person A throws the d20, reads the result, performs any further arithmetic operations required by the rules (eg adding modifiers), and announces the final result.
*People at the table compare the final result to the target number that the rules specify for this saving throw. They note that the final result falls short, and thus the saving throw is failed.
*People at the table - including Persons A and B - therefore update their shared fiction in accordance with the rule I set out three dot points above: they agree that, in the shared fiction, M is ensorcelled to say "Help! Help! The Elves are killing me!"
*Person A, being a theatrical sort of individual, decides to perform, for the benefit of the table, this new aspect of the shared fiction, and so says "Help! Help! The Elves are killing me!"
*People at the table - including Persons A and B - update their shared fiction: it now includes N casting a suggestion spell on M, and it further includes N saying to M, a part of the casting, Say, "Help! Help! The Elves are killing me!"
*People at the table consult the rules - either literally, or via recollection - and note that, in order to work out what happens next - is M ensorcelled? - it is necessary for Person A to roll a saving throw. [We could state the rule as follows: If, in the fiction, one character casts a suggestion spell on another character, then a saving throw must be made by the controller of that second character to determine whether the fiction becomes one in which they are ensorcelled by the spell, or remains one in which they are not.]
*Person A throws the d20, reads the result, performs any further arithmetic operations required by the rules (eg adding modifiers), and announces the final result.
*People at the table compare the final result to the target number that the rules specify for this saving throw. They note that the final result falls short, and thus the saving throw is failed.
*People at the table - including Persons A and B - therefore update their shared fiction in accordance with the rule I set out three dot points above: they agree that, in the shared fiction, M is ensorcelled to say "Help! Help! The Elves are killing me!"
*Person A, being a theatrical sort of individual, decides to perform, for the benefit of the table, this new aspect of the shared fiction, and so says "Help! Help! The Elves are killing me!"
The causal processes here are mostly social ones - people say things to one another, agree on what to imagine together, and identify and apply rules based on that agreement to imagine. The rolling of the d20, though, is a straightforwardly physical causal process. And doing the arithmetic and comparing the numbers is a cognitive causal process, but not a particularly social process.