I am pretty sure the Nazis thought the Jews were rich, because they were rich. They hated them and thought they were trying to take over the world because they were bat-shit crazy, and needed someone to blame for all their problems.
Not quite true and overly simplistic. The Nazis were not bat-shit crazy, they were mostly normal, rational, people who knew how to manipulate society to achieve their ambitions.
After the First World War, a lot of Germans believed in a "stab in the back" myth for their defeat. The basic idea was that jews and other "unpatriotic" and or "alien elements" within German society caused social instability at home, resulting in the german capitulation in November 1918. Early Nazi propaganda associated Jews with the "stab in the back" and with bolshevism, even though it would make almost no sense for "rich jews" to be Bolshevists or to overthrow a monarchy that did a far better job of protecting their minority rights than any other country in Europe.