This can also include any crossovers and circuits connected to the speakers. When the load decreases, the amplifier's output increases. There is less resistance to the current, and the speakers can draw more power from the amp. Drawing more power than the amplifier was designed for will damage the amp.
Every amplifier is designed to handle a certain load.
For home amplifiers this number usually starts with 8 ohms. With car amplifiers it is usually 4 ohms. Most quality amplifiers can also handle a lower resistance. First, make sure that the amplifier you have is bridgable. If you try to bridge an amp that is not designed to be bridged, you can damage the amp.