Modifiers matching inactivity:

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

Simple categories matching unforeseeable:

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