While other responders have doubted the accuracy of your statistics, I would like to rephrase your question and then respond. I am not sure of the current percentage of psychologists who are jewish (I am!!). However, a better question is "how is it that so many of psychology's 'founding fathers' were Jewish?" From Freud, Adler and Horney (psychoanalysts), to Werthiemer (gestalt theory), Beck/Ellis (cognitive therapy), and Yalom (group therapy) many of the major contributors to psychology were/are Jewish.
One previous responder to your query provided a partial answer, Jewish culture strongly advocates for pursuit of higher education. Indeed, a hugely disproportionate number of Nobel prize winners from many fields have been Jewish. Jewish culture strongly values education and acheivement.
However, there is another possible explaination in the case of psychology. Langman (1997) posits that psychotherapy meshes very well with Jewish culture. In his article, he shows how Freud based his rules of therapy on the Rabbi-student relationship in orthodox Jewish culture (he and his wife were decendents of hassidim. While he was decidely irreligious, the author argues that he would have been strongly influenced by jewish teachings based on Jewish living situations in pre WWII Europe). For example, in orthodox Jewish culture, the Rabbi is seen as having special knowledge and many religious individuals go to the Rabbi for advice and share thier most painful memories and dilemmas. The Rabbi dispenses sagacious advice based on his special knowledge and insight. This is similar to the psychoanalist who has "special knowledge" and "procedures" that help him see and interpret the patient's unconcious conflicts.
True, Beck and Ellis' cognitive therapy is much more collaborative and not as similar to Jewish culture, but these were "Americanized" Jewish people who assimilated Jewish culture (talking to a special person) with the more Americanized concepts of science and empiricism (can't see unconcious, so lets not deal with it) and Individualism (let's have a short term collaborative relationship where we deal with a specific problem rather than 2 years of free association and complaining). So even cognitive therapy would mesh with American Jewish culture. I won't summarize the whole article here, but suffice it to say that many of paychology's founding fathers were Jewish and based psychotherapy on principals very similar to Jewish culture. This surely attracted other Jewish individuals to the field who continued to expand on Freud's theories, or developed thier own models of therapy. While I am not sure about the current percentage of Jews in the field (and certainly non-Jews have contributed a tremendous amount as well, such as Skinner and Watson, the fathers of behaviorism). I believe that Psychology closely matches Jewish values of helping others, as well as persuing a higher education. Also, we believe that talking helps, and so the field makes sense to us!.
Langman, P.F. (1997). White culture, Jewish culture, and the origins of psychotherapy. Psychotherapy, 34, 2, 207-218.