This can happen for a number of reasons, and it's hard to say for sure without being able to review your return with you or see your screen.
The most common reason is a reduction in, or loss of, certain tax credits such as the child tax credit (which changes in the year a dependent child turns 17, education credits, or child and dependent care credits.)
These credits are generally lost due to reasons other than an increase in income (change in age or graduation from school), which is why the effective or blended tax rate can vary when they are removed from the tax calculations.
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