@asm_ahsan absent your sharing your AGI ( before and after adding dividend income ) your filing status, all one can do is guess --- you were probably sitting close to the edge of a tax bracket and the dividend income just put your marginal rate higher. Take a look at your form 1040 worksheet ( in the forms mode ) and calculate the tax by hand from the tax tables ( Pub 17 or instructions for form 1040 ).
Why do you say dividend in come should be taxed at 15% ? Is this foreign dividend or what ?.
Sorry , this is probably not the answer you expected , but with your post as the only data point, we are kind of blind.
