River recalled Miranda because River was not herself...
The context is landed out entirely in the opening exposition.
Who is River Tam?
See, most of our best work is done
when they're asleep. We can
monitor and direct their
suggestions. It's a little startling to see,
but the results are spectacular.
Especially in this case. River
Tam is our star pupil.
I've heard that.
She's a genius. Her mental
capacity is extraordinary, even
with the side-effects.
SIMON: Tell me about them.
DOCTOR MATHIAS: Well, obviously, she's unstable...
the neural stripping gives them
heightened cognitive reception,
but it also destabilizes their own
reality matrix. It manifests as
borderline schizophrenia... which
at this point is the price for
being truly psychic.
And later in the conversation:
SIMON: How is she physically?
Like nothing we've seen. All our
subjects are conditioned for
combat, but River... she's a
creature of extraordinary grace.
River Tam is a psychic subjected to behavioral conditioning and neural stripping. More importantly she, like all their subjects, have been conditioned for combat. It is likely that combat conditioning that was triggered in the bar fight.
Who observed River?
Key members of Parliament have
personally observed this subject.
I was told their support for the
project was unanimous. The
demonstration of her power…
River was observed by key members of parliament. Personally! They saw her perform some combat operation/simulation in person when her behavioral modification and implanted suggestions would have been triggered. This is important, when River was observed by parliament, River was not herself!
Who did River observe?
THE OPERATIVE: Key members of Parliament. Key.
The minds behind every diplomatic,
military and covert operation in
the galaxy, and you put them in a
room with a psychic.
DOCTOR MATHIAS: Whatever... secrets she might have
accidentally gleaned... it's
probable she doesn't even know she
knows them. That they're buried
beneath beneath layers of psychosis
River Tam learned of Miranda from one or more key members of parliament while subject to behavior conditioning and suggestion triggers. This is important, when River learned of Miranda, River was not herself!
Her knowledge of Miranda was truly buried beneath layers of psychosis just as the doctor suggests. She doesn't know anything about Miranda, until she is triggered by the subliminal message in the bar. Why can't she remember Miranda beforehand? When triggered by the subliminal message, River was not herself.
The human brain stores information associatively not logically. River's memories of Miranda were associated with her behavioral conditioning. It wasn't until her behavioral modification was triggered that River remembered the previous experiences associated with being in that altered behavioral state, including the secret of Miranda.
River would have never recalled the secret of Miranda until the Alliance triggered her to prevent anyone from learning about secret of Miranda… It is actually quite ironic...