Bottom line is, no one can tell you why except the IRS.
It could be that one or more employers did not submit the employer copy of your W-2 to the IRS. Or it could be that your entries don't match. Or it could be a random verification so they can do some statistical testing on how accurate the system is. Or it could be non-random if you claimed EIC. (A common form of tax fraud in past years was for someone with no job to file a return with just enough fake income that they qualified for a large EIC payment but didn't owe tax.)
