Its likely due to self employment taxes. Your post shows that you are using the Self Employed version of Turbotax. When you have self employment income from a Form 1099-MISC (independent contractor) or general business income from self employment, you need to pay self employment taxes on that income in addition to regular taxes. The tax rate for self employment taxes is roughly 15% (see link below for full description of these taxes) and that does NOT include the regular income taxes on your taxable income. Also, this tax is on your net profit, so itemized deductions or the standard deduction and your personal exemptions etc do not reduce the amount of this tax like they do your income tax.
When you have W2 wages, your employer will withhold these taxes from your paycheck and your employer also is responsible for paying half of the SE tax (whereas you must pay all when you have a Form 1099-MISC). This is a large reason as to why people who have this type of income choose to make estimated tax payments during the year - so they can prepay this amount and avoid a large tax bill.
