Why the four threats and hazards?
The threats and hazards are a reminder why the whole trustworthiness concepts is so essentials: Your industrial system is permanently in danger that something bad happens. In researching what could happen and why we discovered several sources:
(1) the environment which includes the "dangerous" nature with flooding, earthquakes and heavy wind for example but also your supply of energy, water and other necessary sources the system until bad behavior in the neighborhood (for example a large fire)
(2) humans: which includes not only the operators (by doing wrong and probably dangerous decisions) but also the designers (but implementing flaws in the system), visitors (who may be do dangerous things unintentionally) up to customers (abusing instead just using the system). And then there are the humans who plan (cyber-) attacks to your system.
(3) The system itself which may have design flaws, unexpected randomly misbehavior, degrading by using over time without follow up in maintenance etc.
And these sources are creating different types of hazards and threats which are in the center of the four lightning bolts: attacks (by humans but probably also by autonomous systems like botnets or drones), errors (by designers and operators), disturbances (by the environment) and faults (by the system itself).