Rav Moshe Wolfson says that having payos behind the ears circumvents a kabalistic problem of mixing beard hair with payos hair, though I would imagine he admits that the direct cause for the change (payos behind the ears, or not at all, i.e rav chatzkel) was anti semitism.
It’s for the above kabalah reason (according to rav wolfson) that the minhag in chabad is not to have long payos.