Second job or raise would do it. You were making $x and paying y% in taxes. Now, you are making more money and paying z% in taxes. So, the money you made at the lower income did not have as much tax taken out as necessary. If you have a second job, those will never withhold enough and you have to have more taken at your main job. On the W4, there is a box where you can withhold additional money, if needed.
Good answer by @Mkeithddc. When your income goes up, so does your tax bracket.
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