The proximate cause, as ever with any kind of bigotry, is of course sheer stupidity. And fear of difference, of the other, obviously.
The ultimate cause - well, a great many people would like to know that. Poorly disguised anti-Semitism hiding behind concern-trolling criticism supposedly of "Israel, but not Jews" is certainly one factor amongst many in my view. A drip-drip effect of anti-Semitic propaganda such as you can find on forums such as this one is another.