Since people hate Jews, the word Jew is often used in a derogatory way. Then people start associating Jew with derogatory things.
[search wikipedia: “Jew (word)”]
This happens with names of other targeted minorities which are considered respectful at one point, but then become seen as derogatory.
Similar to the word shvartze which is just Yiddish for black, but many people consider offensive because they might have heard it in an offensive context and assumed that it was derogatory.