It is a common mistake to think that the bull is charging at the red cape, because it is red. In all actuality, bulls are color blind. The cape is red so that it can hide the blood spatters later on when they spear the bull repeatedly and kill it. The bull is actually irritated by the movement of the cape.
The red cape is actually one of many capes used dutring the fight, and it's the last one used. Before, there are several other capes of different colors to taunt the bull into charging. The red one is brought out when they're really ready to spill some blood and kill it.
