It could be some settings in the AIW file, settings in the talent file, and/or settings in the player file; they all interact, the plr file settings affect all AI at all tracks, the talent settings affect that AI in any car at any track, the AIW affects any AI at that track. For a good overview of adjusting the AI, get the Von Dutch AI Tutorial in the library here (it applies to GTL and rfactor also, though actual values vary).
A higher aggression setting will also make them run closer together and attempt more passes, but may also cause more contact (which leads to aero damage, which is cumulative, and eventually causes them to slow down; AI will not pit to repair this unless they get a certain amount of damage in one hit). There are threads here discussing this issue and how to correct it.