My guess is that he is really, really bored, and doesn't yet have the maturity to get it out of the way so he can do something else.

Can you make his work harder? Ask him to focus on something else- like how many noses are supposed to be colored blue? I'm not sure how to make letter tracing tricky. If I could satisfy myself that my kid didn't really need the tracing practice I'd be tempted to let him skip it.