you won't find the answer here. this is an income tax forum anyway, it's the 3 rating companies that compute your score based on various factors and only they know what changed to lower your score. but here are a few of the possible reasons:
did you finance or refinance a home or vehicle
have you been paid late or missed a payment
did you request a credit limit increase
your credit utilization factor has increased. it's is the amount you owe on your cards vs. your credit limit.
did you close a credit card account
did you loose your job