Answer by taking a quote from the following article:

Ellington was born on April 29, 1899, to James Edward Ellington and Daisy (Kennedy) Ellington in Washington, D.C. Both his parents were pianists. Daisy primarily played parlor songs and James preferred operatic arias.

From 1936, Ellington began to make recordings with smaller groups (sextets, octets, and nonets) drawn from his then-15-man orchestra and he composed pieces intended to feature a specific instrumentalist, as with "Jeep's Blues" for Johnny Hodges, "Yearning for Love" for Lawrence Brown, "Trumpet in Spades" for Rex Stewart, "Echoes of Harlem" for Cootie Williams and "Clarinet Lament" for Barney Bigard. In 1937, Ellington returned to the Cotton Club which had relocated to the mid-town Theater District. In the summer of that year, his father died, and due to many expenses, Ellington's finances were tight, although his situation improved the following year.  After leaving agent Irving Mills, he signed on with the William Morris Agency. Mills though continued to record Ellington. After only a year, his Master and Variety labels, the small groups had recorded for the latter, collapsed in late 1937, Mills placed Ellington back on Brunswick and those small group units on Vocalion through to 1940. Well known sides continued to be recorded, "Caravan" in 1937, and "I Let a Song Go Out of My Heart" the following year.  Billy Strayhorn, originally hired as a lyricist, began his association with Ellington in 1939. Nicknamed "Swee' Pea" for his mild manner, Strayhorn soon became a vital member of the Ellington organization. Ellington showed great fondness for Strayhorn and never failed to speak glowingly of the man and their collaborative working relationship, "my right arm, my left arm, all the eyes in the back of my head, my brain waves in his head, and his in mine". Strayhorn, with his training in classical music, not only contributed his original lyrics and music, but also arranged and polished many of Ellington's works, becoming a second Ellington or "Duke's doppelganger". It was not uncommon for Strayhorn to fill in for Duke, whether in conducting or rehearsing the band, playing the piano, on stage, and in the recording studio. The 1930s ended with a very successful European tour just as World War II loomed in Europe.

Did anything specific happen in the later 1930's for Duke?
From 1936, Ellington began to make recordings with smaller groups