The odd one word is  

Why? 

"crack" refers to solids; "spill" and "flood" both involve liquids.

The odd one word is  

Why? 

"break down" means fail mechanically; "break" and smash" both mean break physically.

The odd one word is  

Why? 

"leak" refers to liquids; "come off" and "crack" can both refer to pieces falling off an object.

The odd one word is  

Why? 

"stain" means "leave a mark"; "fall down" and "bump" both refer to ways you can injure yourself.

The odd one word is  

Why?