Made a post to r/vexillologycirclejerk a few weeks ago after discovering that the flag of Kashubia is just the flag of the Habsburg monarchy. What connection, if any, do the two flags share?
It is a coincidence. The Kashubian flag's colors are basically the colors taken from the Coat of Arms, which is a black griffin on a yellow background. And I guess the Habsburg flag's colors are also taken from their Imperial Eagle.
However you could find a root in this similarity. Nominally, the Holy Roman Empire's Coat of Arms has historically been a black eagle on a yellow. The Habsburg Eagle derives from the HRE's one, and the Kashubian griffin's colors are also based on the Medieval Imperial Eagle.