1. In a way that was not expected
   - out of the blue 
2. Without advance planning
   - by chance, accidentally, circumstantially

See also: unexpected

• superphosphate
• superphylum
• superposable
• superpose
• superposition
• superposition principle
• superpower
• supersaturate
• supersaturated
• supersaturation
• superscribe
• superscript
• superscription
• supersede
• supersedeas
• supersedure
• supersensitised
• supersensitive
• supersensitized
• supersession
• superset
• supersize
• superslasher
• supersonic
• superstar