Many things cause a bike tire to deflate. Glass, sharp rocks, tacks, and nails can pierce the tire and puncture the tube within. A tube can be pinched between the rim and tire causing the tube to split when inflated. If a tire has a hole in it, the tube, which is filled with air pressure, will bulge out of the opening and pop. As well, the valve holding the air pressure in the tube can be damaged or faulty.