What are some common reasons I might have developed gallbladder polyps?

See below. Gb polyps affect 5% of the adult population. Exact cause is not known but related to age and presence of gallstones which in turn is related to obesity as well.
Bad luck. What size are they? Speak with a general surgeon about this. Take your ultrasound report so they can evaluate the size and characteristics. I would not ignore this without first discussing with a surgeon.