Here's what others have said: "The definitive cause of uterine polyp is not known. However, they seem to be caused by hormonal levels and usually respond to estrogen circulation by growing. Endometrial polyp often cause no symptoms. In the event that they occur, symptoms will include bleeding between menstrual periods,irregular menstrual bleeding,vaginal bleeding soon after menopause and menorrhagia; a condition characterized by excessive menstrual bleeding. When polyp blood vessels bleed, there is an increase in blood loss during and in between menstrual periods. In the event that polyp protrudes into vagina, pain would result." http://uterinepolyps.net/tag/endometrial-polyp-2/
The bigger question: "How do they relate to endometriosis?"
Interestingly, endometriosis patients are much more likely to have them. "In a retrospective study, examination of 431 infertile women (158 cases with endometriosis and 273 without endometriosis) showed a significantly increased frequency of endometrial polyps in patients with endometriotic infertility and no significant differences among different stages and locations of endometriosis." http://www.fertstert.org/.../S0015-0282(11)00680-7/abstract
Enough so that they:
"We strongly recommend hysteroscopy if endometriosis is detected in a woman undergoing evaluation for infertility, even if hysterosalpingography and transvaginal ultrasonography do not suggest endometrial polyps." http://www.journals.elsevierhealth.com/.../S1074.../abstract
"The presence of multiple endometrial polyps seemed to be associated with the presence of adenomyosis (p=0.016). The presence of cervical polyps was significantly associated only with presence of adenomyosis (p=0.002). The results clearly demonstrate an association between adenomyosis and uterine polyps. The pathogenetic role of adenomyosis in the development of polyps should therefore be investigated further." http://www.ncbi.nlm.nih.gov/pubmed/21470766