I was curious about this too. The most information I can find is from a Sounders fan's explanation on Reddit:
This tracks with the fact that Seattle was fined for misappropriation of credentials. So I'd say the penalty for Lenhart is so harsh because he wasn't appropriately credentialed as a coach to be on the sideline, and then he went out onto the pitch and got involved. Combine that with no CBA/Union to protect coaches, and his penalty is by far the harshest.