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

contact the reporting agency.  they will be the only ones to know why.  as i said this is a tax forum.  we have no access to your tax return or credit reports.