You are putting a needle into your body, where there are tiny blood vessels. The capillaries and veins can be damaged by the needle insertion, if you hit them. Plus, when you inject, you can cause a small trauma to the area if you go too fast. Both these things could cause bruising. You are more likely to have bruising if you are on aspirin therapy, or taking any form of blood thinner.
With syrnges, you can attempt to draw back on the needle before delivering a shot. Make sure there is no blood in the syringe before injecting, so you are certain you are not in a vein. This will NOT assure that you did not pierce a vein on your way in or out.
Hope that explains it somewhat. HUGS! sheryl