It is quite common for this to happen for a number of reasons. Your tax liability is calculated on your total income (after deductions and credits), not on each W2. As your taxable income increases, your tax liability increases and you may even get placed into a higher tax bracket. Depending on your income before, the income from roughly $75,000 - $150,000 was taxed at 25% compared to 15%.
As your adjusted gross income increases, you may also lose eligibility for certain deductions and credits. And you also have to factor in the fact that either your standard/itemized deductions and personal exemptions were being applied against the initial income (if married filing joint with no dependents taking the standard deduction, the first $20,700 was not being taxed at all). So, it is likely that all of that additional income was taxed at not only the higher rate of 25% but also it didn't have the benefit of your deductions.
I have included an article below that has more detail on this topic.
