There are a few different things that can cause a sudden flat tire. If you hit something sharp, like a nail or piece of glass, it can puncture your tire and cause it to go flat. Another possibility is that your tire has developed a leak. This can happen if the rubber gets old and cracks, or if the metal rim becomes bent or damaged. Finally, if you've been driving on bald tires for awhile, they may eventually blow out from all the wear and tear.