I think the reason it changed is probably deeply bound up with the rise of fundamentalist Wahhabism and Salafism in the 18th century.
I think the rise of antisemitism accompanies the emancipation of Jewry in whatever society it has lived. I think that most societies across the world in which Jews have found themselves living (and some where no Jews have ever lived) have inbred anti-semitism which is latent, as long as Jews are poor and subservient. This certainly apples to Jews in Arab countries before 1948 and Jews in Germany until approximately the middle of the 18th century.
Antisemitism becomes widespread and open at about the same time as Jews begin to demand and receive treatment equal to the societal majority, or, even worse, form their own ruling class.
German antisemitism became widespread, popular and acceptable shortly after the widespread emancipation of German Jews at the turn of the 20th century. Arab antisemitism flourished immediately after the establishment of the State of Israel. Up until that point most Arab nations tolerated, to some extent, a Jewish minority, of greater or lesser extent. As soon as Jews as a class demonstrated to Arab society they had the skills to win wars, form governments and form a ruling class - that those skills and positions were not by breeding or inclination forever beyond Jewish reach - anti-semitism boiled over in a significant way, leading to the expulsion of all Jews from those nations.
I see echoes of this the world over: Jews are ok as long as they don't win anything, rule anything, own anything, or manage anything. Quiet, invisible Jews who know their place are acceptable, just about. The others, the bad Jews, the ones who get ideas above their station as Jews, have to be knocked down.
The real crime of Israel, in the eyes of the left, is to be Jews who win.