In this instance the motivator, the fox and chasing it was stronger than the motivation to come to you when called. Chasing furry things that move is an awfully strong instinct in dogs and it is not possible to get 100% reliability. There's always a chance that a dog will obey his instincts over you.Your dog has been well trained and well motivated for the most part. This time, for some reason the urge to chase over came him. If you read Culture Clash, there's a part in there which describes in detail how to train for such things to improve the reliability. I think it was in Culture Clash. I've read a lot of books and sometimes I get them mixed up....pretty sure that was the one. Excellent read anyhow. I recommend it.
My Doberman has been very reliable too when chasing animals; deer, rabbits, squirrels and other dogs. One time though, he didn't "hear" me when he was after a bear. Boy, did that freak me out. Luckily bear can outrun a dog and he did come....just not before he got the bear to leave the property. Grrrrrr. Scary! My male Chihuahua did the same thing a couple of summers ago. The bear ran for his life, let me tell you.