Assuming nothing else changed, there's a few possibilities.
As @binarymax mentions, your credit utilization ratio may have dropped. You mention in the comments that you have zero debt, but if you have any credit cards the ratio is typically calculated from the statement balance, I believe. I've noticed my FICO can fluctuate because of that, even though I pay off all my cards in full every month. Of course, if you have no other accounts, you'd have a 0/0 utilization. I don't know how the Bureaus handle that, but 0/0 =/= 0...
Second possibility is the number of accounts. If you have vary few accounts, dropping one may be enough to significantly impact this portion of the score (though it's a small portion IIRC..)
(and IMO most likely), dropping the student loans probably dropped the average age of your accounts. I would expect that for many people with student loans, they are one of their oldest credit accounts, if not the oldest. Age of accounts is a pretty significant factor in the FICO calculation, and it's quite possible that it just dropped by a couple of years.