What exactly causes clicks and pops on vinyl?

At the time of pressing, release agents are used that can/will collect dirt from the atmosphere and the cheap paper liner records come in. That's why it's good to use one of those nice record cleaning machines before ever playing the disc(bought my VPI back in the day for $325.00). When your stylus winds it's way through the groove, it actually melts the surface just as skates do to the surface of ice(just talking a few molecules here). If not cleaned prior to play- particles of dirt will be welded to the groove walls. These of course make noise and wear your stylus. Before I ever played a record(usually tried to find virgin vinyl pressings, MFSL, Crystal Clear, Sheffield Labs, Telarc, etc), I'd use the VPI, treat the record with L.A.S.T. after drying, let that sit awhile and finally: use my Discwasher brush on it. My records are stored in rice paper liners. Cleaner than paper, and they don't leach the elastomers out of the vinyl. Sound anal? The records I purchased over twenty eight years ago are still pristine, and I've gotten excellent mileage out of my styli. At very least: find a Discwasher brush and liquid, clean your discs b4 each playing, and don't scratch(OUCH) or touch the grooves with your fingers as Pbb said.