Why does my unfixed dog seem to be targeted by fixed dogs?

So it has happened to my male unfixed dog on more than one occion where we are at the park or walking down the street and another dog who is fixed seems to target him either by growling or trying or actually attacking him. He is very pive and will not retaliate unless attacked (biten at) first . I have heard that fixed dogs will target unfixed dogs. is this true? I know dogs can be conditioned and trained to not be aggressive which he has been. My question is if anyone knows if a fixed dog has a natural tendency to attack an intact dog? It seems like he is being targeted