It appears to be a musical "non sequitur," an anomaly.
The following year, when Zaremba joined the faculty of the new St Petersburg Conservatory, Tchaikovsky followed his teacher and enrolled but did not give up his post at the ministry before his father consented to finance his further studies.
From 1862 to 1865, Tchaikovsky studied harmony, counterpoint and the fugue with Zaremba, and instrumentation and composition under the director and founder of the Conservatory, Anton Rubinstein.
Though a brilliant composer, his life was riddled with sadness.
Pyotr Tchaikovsky was born in Votkinsk, a small town in the Vyatka Guberniya, now Udmurtia (a sovereign republic within the Russian Federation) to a mining engineer in the government mines, who had the rank of major-general, and the second of his three wives, Alexandra, a Russian woman of French ancestry.
In the first movement, the rapidly progressing evolution of the transformed first theme suddenly "shifts into neutral" in the strings, and a rather quiet, harmonized chorale emerges in the trombones.
The trombone theme bears no resemblance to the theme that precedes or follows it.
Tchaikovsky noted later that he was fortunate not to have been brought up in a very musical family that would spoil him with music imitating Beethoven.
He received piano lessons from a freed serf, beginning at the age of five, and within a few months he was already proficient in Friedrich Kalkbrenner's composition Le Fou.
He made an acquaintance of the Italian master Luigi Piccioli, who influenced the young man away from German music and encouraged the love of Rossini, Bellini, and Donizetti, whom he had listened to as a child.