What causes someone to have bipolar i with rapid cycling ?

Combined factors: The best we can tell, it's a combination of genetic vulnerability and ongoing biopsychosocial modulation. People may have individual triggers for episodes, and discovering & dealing with those is part of the work one does with one's own doctors.

Bipolar: Could be genetic or effect of psychosocial factors.

Genetics & triggers: Rapid cycling bipolar d/o involves 4 or more discrete affective episodes in a year. It is more frequent in women, associated with hypomania, hypothyroidism, stronger family hx, & higher exposure to antidepressants (though it may be depression itself that leads to both rapid cycling & increased antidepressant use). Careful evaluation & management by a skilled psychiatrist is essential.