Modifiers matching inactivity:

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

Simple categories matching unforeseeable:

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