...if anyone knows whether there are any reasons why it seems to stray somewhat from the Nocturne formula, e.g. biographical or other ones.
Wiki tells us much more about this nocturne - what you may probably know:
The Nocturne in C minor has been categorized as one of Chopin's greatest emotional achievements. Theodor Kullak said of the piece, "the design and poetic contents of this nocturne make it the most important one that Chopin created; the chief subject is a masterly expression of a great powerful grief." Jan Kleczyński, Sr. calls the nocturne "broad and most imposing with its powerful intermediate movement, a thorough departure from the nocturne style." Some musical critics, including Charles Willeby and Frederick Niecks, do not think the piece deserves its fame and position; though James Huneker agrees with this assessment, he notes that the nocturne is still "the noblest nocturne of them all." James Friskin found the music to have "the most imposing instrumental effect of any of the nocturnes," calling the crescendo and octaves "almost Lisztian."
In another source we can read:
The two Nocturnes op. 48 were completed in the autumn of 1841. They are dedicated to a favorite student of Chopin, Laure Duperré. One of the composer's major works, Nocturne op. 48, No. 1 in C minor (Lento), is an impressive masterpiece of the tragic force of his power. His gloomy and austere solemnity is associated with an intensity that is sometimes almost unbearable.
It is one of the longest and most dramatic nocturnes. It is a real diary of Chopin, in which one can guess the expression of intense pain. Chopin wanted the first measures to emerge as a thematic element. As such, it accentuated the sound of the first three notes by playing with the third finger, the most sonorous finger on the piano.
This first episode is "Lento" is followed by a new point "Poco più lento" with peeled and arpeggio chords that bring as hope, but gradually wins agitation, the volume increases, as numbers obsessed with triplets and sixteenth notes in octaves and gradually accelerating towards the last passionate passage "Doppio movimento" feverish and desperate. The theme is striking, on a low contour more romantic and tortured. The night ends as a long lament.
In the article above we learn a lot of Chopin’s biography, his love with the 16 years old Maria Wodzinski 1838, his illness 1838 and his relationship with George Sand and the dying of his father in May 1841.
But we find no evidence of influence to op. 48 written in October 1841.
Nocturne 1-10 have been composed before 1838, op. 55 and 62 1844 and 1846.