This is an important question. It happens to many of us and can be caused by multiple factors!
Two of the most common causes are:
1. Lack of consistent flossing which allows for gingivitis (infection of the gums) to develop. This means that when you do floss the gums bleed due to the bacterial load in the gums. When the gums are infected, all the blood vessels dilate and rush to the surface to help heal the infection of already tender, inflamed gums, and flossing causes bleeding as a result. You should continue to floss regularly and will see overtime a gradual decrease of bleeding upon flossing. This is an indication that your gums are healing as a result.
2. Mouth breathers tend to have more inflamed gums and therefore on flossing the gums bleed. If this is the case, you need to address the reason you are breathing through the mouth at nights and allow yourself to sleep with a closed mouth. When this is addressed, you will see a decrease in bleeding of the gums when flossing.