Is bicycling likely to cause hip bursitis? What about bicycling daily with a backpack weighing ~20 pounds? I bike ~8-12 miles per day, 6 days/week with such a backpack. Thanks.

A thin person who- -wears tight fitting cycling pants can very well cause trochanteric bursitis. It often happens 2 those who wear tight jeans. It is a more likely cause than the back pack, but the extra weight causes U 2 work harder & that can B a contributing element, Pants, shorts, ,or mid-length. It's the tightness @ the hips. .
Wear and tear. Any activity would create wear and tear effect unless you had a previous injury biking with or without backpack should strengthen your pelvis muscle and protect your hip if you are having problem with your hip have it checked before you resume vigorous activity because you might aggravate an injury good luck.