If the piping is galvanized, its caused by calcium and mineral deposits on the inside of the piping. The ions in the steel and the ions in the calcium and manganeese are oppositly charged. So the iron pipe draws the minerals out of solution to block up the piping. Just like calcium and fats block up the arteries in your heart with high colesterol. The reason the hot blocks up first is because the heat makes the piping more attractive.
If your piping is copper, then there is some debris in the piping at the faucet or at the angle valve coming out of the wall.