Toyota in the Big 3?

Ever since I can remember the biggest three automobile manufacturers in the Unites States have been GM(Chevy, Pontiac, GMC,...), Ford, and Chrysler(Chrysler, Dodge, Jeep,...). However, now somebody else has crept their way onto the scene. Some expect Toyota to pass GM within the year to be the number one manufacturer world wide. They are also saying that it is not a question of if Toyota will pass Chrysler, but when Toyota will pass Chrysler as the third largest manufacturer in the United States. So what could be causing this change? Could it be the continuing rises in gas prices that shifts the market to a much less gasoline using brand of cars, could it be that Toyota is making a better automobile than the others, or is it that the other automobile manufacturers are slacking off and not making as good of vehicles as they used to?