What causes a person to get a risen?

I will assume you are speaking of a cyst. Many things can 
cause the formation of a cyst. They can be infected or not infected.  A good example of
a cyst is in the female who shaves her underarms or axillary area. Sometimes when women
shave this area a small ingrown hair will be the initial site of the cyst. The tissue is
irritated by the hair and the body responds by inflammatory processes which causes the
area to swell. Pain in the area may be the first symptom, it may also be red, tender to
the touch, and heat may be felt in the area. Some of these will be infected, others will
not. With a cyst in this area of the female it is important to determine exactly what
the growth is because some forms of breast cancer can spread to this same area and may
be felt as a cyst.

I would encourage you to go see your
primary care physician to determine the etiology of this growth.