25 y.o.One wisdom tooth partially erupted since 2010.somedays the gum around and above this tooth suddenly starts bleeding for a while. Why?

Inflammation. If the gum bleeds, it is inflamed. Food or bacteria being trapped.
Needs Rx. Bleeding is a sign that something's wrong. Usually infection, inflammation, or trapped food debris. Partially impacted wisdom tooth can be doing damage to adjacent tooth or tissues. See your Dentist or preferably a specialist Oral Surgeon for evaluation and treatment.