DMertz offers this explanation:
Do you mean that you yourself are calculating a marginal tax rate of 54% on a certain amount of self-employment income? While not common, a 54% marginal tax rate on the addition of some amount of self-employment income is entirely possible due to side effects of the increase in your AGI from the self-employment income even though your ordinary income falls in the 10% tax bracket. Compare the details of your tax return with and without the additional income to see what is contributing to the change in tax.
