Because that happened to my nose
If you wank too hard
Probably weakness of the vein wall due to it being a varicose vein and/or something constricting the vein preventing the return flow of blood.
on my left ankle my vein burst through my skin and was bleeding every where,comming out like a fountain.
There are many factors that can cause this. Mostly hardened blood vessels or thin walled blood vessels, usually paired with high blood pressure, sometimes a persons blood vessels can be thin enough...
Cleaning your belly button or scratching your arm pits.