Nearly all of Bach’s keyboard concertos were afterthoughts.
As much as the piano has dominated the concerto genre in the last 230 years or so, the keyboard came late to the ranks of concerto soloists. Even Vivaldi, who wrote concertos for nearly every other instrument, including quiet instruments like the lute, never wrote one for harpsichord.
(The pianoforte, which until after Bach’s death was just an unusual variant of the harpsichord, was an even less likely concerto soloist candidate. Bach once tried a famous builder’s early attempt at making one, and told him the instrument’s treble notes were weak. Of course, the modern piano, vastly louder than any harpsichord or piano Bach would have known, is another story.)
Bach may have been the first to write harpsichord concertos, and even those concertos are more rewritten than written. With only two likely exceptions (including the Fifth Brandenburg), his 15 concertos for harpsichord are likely reworkings of compositions for other instruments. We don’t know why he made these arrangements, but they could have been training pieces for his own sons. Both concertos on this program are in all likelihood arrangements of violin concertos.
The D-minor Concerto shows how diligently Bach studied Vivaldi: the statements of the theme in unharmonized octaves that begin the first two movements and the aggressive cast of the themes in the outer movements are typical Vivaldi touches.
The first movement of the F-minor Concerto is unusual in setting the soloist off from the orchestra rhythmically, with the orchestra playing in straight 2/4 time and the soloist playing almost exclusively in triplets. The second movement is an arrangement of the Sinfonia to Cantata 156, Ich steh mit einem Fuß im Grabe (I stand with one foot in the grave), which means the movement is actually an arrangement of an arrangement.