Gotta love when your credit score keeps declining every week because you keep paying off your student loans like a boss. Student loans are considered "good debt" (LOL) and paying them off decreases your account age, removes variety from your credit mix, and removes the possibility of building an even longer payment history on those loans. Having older accounts, multiple accounts, and a healthy mix of credit and loans are good for score, and paying off student loans means you take a hit to all three of those things. Not to mention the years of payment history you most likely had.
Luckily, it's not always a drastic drop, and who the hell cares? You're paying off your student loans!
"We're going to lower your credit score for being proactive and not being thousands of dollars in debt. Congratulations, go $#&@ yourself! :D"