My dog hates Shelties?

I have a rescue border collie cross and my family and I often take him to local dog shows. He is very submissive to other dogs and he absolutely loves to play with other dogs. He is never aggressive, even if another dog is aggressive to him. However, I noticed that he is extremely aggressive to shelties/ miniature collies. If he sees one even from across a whole field, he will bark, snarl and growl as if he were aggressive. He does not know any shelties apart from one that he sees on a walk occasionally. Even at his previous home he did not have a sheltie living with them. This is extremely annoying as at dog shows there always seems to be a lot of shelties! Any ideas of why he acts this way and how to stop it?