"Don't cry because it's over. Smile because it happened." -Dr. Seuss
Question #84603 posted on 11/19/2015 7:08 p.m.
Q:

Dear 100 Hour Board,

Why are cats afraid of cucumbers?

-Sheebs

A:

Dear Doctor,

Because they're scary.

-Tally M.

A:

Dearest Sheebs,

It took me 46 hours to understand this. Before then I just thought you were crazy. I'm glad you're not crazy. 

Love,

-Auto Surf

posted on 11/20/2015 10:03 a.m.
It's actually most likely because:

a) They look like snakes
b) As ambush predators, cats are not used to being snuck up on
c) The cucumbers were all placed behind them while they were eating (I.e., in a vulnerable position in a vulnerable place during a vulnerable activity during which they expect to feel safe.

Think "Someone Put A Fake Spider On The Toilet Paper" and you're got the general idea.