If the amplifier is continually driven too hard, the amplifier's protection circuitry will kick in to prevent damage to either the amplifier or the speakers.
The protection circuitry monitors things like clipping and temperature, and if you continually drive your amp into clipping or into high temperatures then the protection circuitry will activate and either turn the volume down automatically or switch the amplifier off before any damage occurs.
If the CAP5 protection continues to cut in, it is advisable to check your speaker connections to ensure that there are no loose strands that could cause a short circuit.
It is also advisable to check the specifications of your speakers to ensure that they are compatible with the CXA61/81. Both amplifiers can be used with speakers with an impedance of between 4 and 8 Ohms.
The CXA61 outputs 60W RMS into 8 Ohms and 90W RMS into 4 Ohms, while the CXA81 outputs 80W RMS into 8 Ohms, 120W RMS into 4 Ohms.
If your connected speakers fall outside of these specifications, then this could also be a reason for CAP5 protection being activated, as this can cause the amplifier to be driven too hard.