Because the "anti"-fascists are fascists and PEGIDA are anti-fascists. Their anti-fascism is one of the reasons why they are against Islam -- anyone who is against suppresion, violence, and fascism ought to be against Islam, just as they should be against Communism and Nazism.
I don't think you can call PEGIDA antifascists, since they're, well, far-right nationalists who hate immigrants and have violent tendencies. Just because the Antifa organization holds an intellectually inconsistent position doesn't mean the reverse position from Antifa is actually correct.