Certain components in the engine compartment can be very hot, e.g. the engine, the radiator and parts of the exhaust system.
If you open the engine bonnet when the engine has overheated or during a fire in the engine compartment, you could come into contact with hot gases or other escaping operating fluids.
Windscreen washer concentrate is highly flammable. It could ignite if it comes into contact with hot engine component parts or the exhaust system.