Speaker impedance always varies with frequency. Speaker impedance may be much lower than the rated "nominal" impedance at certain frequencies. The over-current fault detection circuits prevent potential damage that can be caused by driving very high signal levels into very low impedances. In STEREO mode, at full output, the current limit can be reached if the load impedance dips below about 1.3 Ohms. In MONO mode, at full output, the current limit can be reached if the load impedance dips below about 2.6 Ohms.
This is from the AHB2 manual... page 18