Part of your credit score depends on the mix of credit that you have. Installment loans, such as an auto loan, are a "better" type of credit than credit cards are. So, by paying off a better type of credit, the mix of your remaining credit becomes worse, causing your credit score to drop.
Your personal financial situation has improved, but your credit score dropped. The lesson to be learned here is that what is good for your credit score and what is good for your personal financial situation are not always the same thing. People should spend less time worrying about their credit scores and more time worrying about their basic finances.