Why a bad tire gets so hot?

[Inspector71]

Yeah, I know, I've driven longer on my tires than I should but new ones are so expensive and I wanted to get all four at once. I only needed about another week. Left rear tire started going flat. Put air in it two weeks ago and now only an almost bald tire. Got home from work and could smell burning rubber, my tire last night. Left for work at 0-dark-thirty and could hear and feel the problem. Made it to a gas station with free air. I could put air in to some degree but it would not go back to normal. The tire was so hot I could I could barely touch it. Made it home. What makes a bald going bad tire get so hot? Maybe a dumb question but just wondering.