Your long-idle card is now reporting a balance, so to the FICO scoring model it looks like you've recently started using a dormant credit account. "New activity" on a previously dormant card is interpreted by some as a sign of money difficulties, in the same way that opening new credit accounts is sometimes a red flag.
The recent activity on an unused card can result in a small decrease in score. This drop is often just temporary. The good news for you is that your score is still very good, even with the drop.
As an aside, you might want to consider using that card at least every few months. Credit card companies are cutting credit limits and closing accounts like crazy, and they typically target unused credit cards, since they lose money on these accounts. Since you pay an annual fee, then maybe your card won't be high on the hit list, but why take the chance? 21 years would be a lot of credit history to lose, as would be your $17,000 credit limit.
