Cables and connectors are # 1 likely cause, of course.
Don't discount electrical noise from cell phones, fluorescent lighting, air handlers, blowers, electric doors, subways, golf carts, alien craft, etc.
Close walk-bys with nylon clothing can create static pops as well.
Don't let gals in spandex swing on your mic pole. (was going to make this gender inclusive, but wasn't sure how to word it)