Modifiers matching inactivity:

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

Simple categories matching unforeseeable:

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