Modifiers matching inactivity:

• passive,
• physical,
• quiet,
• unforeseeable,

Simple categories matching unforeseeable:

• disaster,
• cause,
• reason,
• event,
• situation,
• circumstances,
• setback,
• risk,
• disturbance,
• occurrence,
• calamity,
• incident,
• condition,
• circumstance,
• change,
• contingency,
• catastrophe,
• inactivity,
• thing,
• development,