95% of the time that is caused by a liquid spill. It can be in the form of syrup dried out causing the key to physically stick in a down or slightly down position, OR by liquid having migrated into the membranes below the keyboard and electrically shorting things out, causing problems even when no keys are pressed.
Often if one key on the keyboard is stuck, it will cause other keys pressed after it to begin repeating themselves instead of the key that is actually stuck. This can reset itself when the computer or keyboard are rebooted, and lie in wait until any key (or a specific key) is pressed, then start the repeat behavior.
