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
Credit dropped 70 points and don’t see why. Wondering what is holding it back and how do I raise it?
You should first obtain your credit reports (free of charge) from the link below and review them carefully.
You can check the link below for information on how to raise your credit score.