Shalom
The reason why "many" (which is a myth) Afrikaaners are anti-Jewish, is because of the self hating leftist-jews. Remember, South Africa fought communism on the Border War, and today, thousands of Afrikaners are being murdered by communists. Now you see, the anti-jews have told the afrikaaners, that the Jews are behind communism, and the Jews are behind the genocide against the Afrikaaners-and all the rubbish crap.
Another sad reason, is because Afrikaaners see the StørmFrønt as the only real srong organizaton out there, fighting for the survival of the white man. And so, some join up (and the only reason they sign up, is because of the blacks), but sadly then get indoctrinated by the anti-Jewish snakes!
But the fact is, MOST Afrikaners and Boers, are pro-Jew/Israel. For instance, Boers are strong believers in the Tanach/Torah-even more than the New Testament. Most Boers are circumcised at young age, and many other G-D fearing believes.
There might be a very little amount of anti-jewish Afrikaaners, whom are making a bad name, and so, many think that "MOST" Afrikaaners are anti-semitic-but it's all rubbish
Hope this helps?