I've witnessed it with my albino marcianus when they were only 5-7 months old. It's like they are bobbing and weaving, it reminds me of boxing a little bit. William is correct in what he said, It's like they're sizing you up. They also do this as a bluff technique to make themselves appear more dangerous than they really are.

If the snake were much older and has been fed a steady diet of goldfish or another unsafe fish, then it would be a cause for concern as it would indicate a thiaminase problem.