Ampacity adjustment is required when there are more than 3 current carrying conductors in a raceway. If you look at table 310.16, you will notice that that is what the table values are based on. After that, see 310.15(B)(2)(a).
When you have a single conductor carrying current there is energy lost in the form of heat. The more current, the more heat. A bare wire can handle a lot of current without any problems for the bare conductor itself because of the high melting point of the metal conductor.
But when you need to insulate the conductor from other conductors, the insulating material usually has a much lower melting point than the bare conductor itself. The insulations lower melting point becomes the limiting factor.
Taking several insulated conductors and placing them in close proximity effectively places several heat producing sources close together. Less of the heat that is generated in each conductor can disipate into the surrounding enviroment causing each conductor to run at a higher temperature. These higher operating temperature conditions combine to make it so each conductor can not carry as much current without aproaching the safety point before melting the insulation.
Shane's description is very good. I only want to add that it is usually not the melting point of the insulation that is the limit. Rather, it is the temperature which will degrade the material over a very long period of time. The melting point of PVC is at least 150C, but it will become brittle by exposure to only 100C for a long period of time.
[This message has been edited by C-H (edited 07-12-2004).]