Some of the people behind the anti-Israel boycotts are anti-semitic or motivated by desire to bond with Arab interests. Then a lot of people just hear about issues and want to help the Pals. If the Jews were being cornered and had their rights to live a normal life infringed on, they would want advocates.
Yemeni ISIS affiliate claims responsibility for Sanaa mosque bombing (Reuters)
from the article: How hostile is Britain to Israel?