This is just the black bleeding into the Yellow and there are several things you can do.
1. Some shirts exhibit this more than others, so you can try a better quality shirt.
2. Slowing down the print speed (for example using Uni directional) will reduce bleeding.
3. Using a white shirt pre-treatment holds the ink better and reduces bleeding.
4. Reduce the MaxInk value (go to job properties and change the Maxink value to a lower value), this will change the CMYK black total ink limit
5. Print using just Black ink (although this may not be black enough), you can do this making making sure the artwork is CMYK and then making the black just K. In the job properties go to the ICC tab and click on the down arrow at the end of the CMYK lines and click on Preserve pure black