What causes "dieseling"

TT5.9mag said
@Princess Valiant shortbed? Fleet side? Wanna sell it if both of the answers to the above questions are yes.

It started life as a step side but either the factory or someone put the tool boxes on, which is the main reason I bought it because I wanted a tool truck because I am too lazy to lug heavy tools everywhere I go.

I'm not sure what a fleet side is but it is a shorty.

At this time I am not selling, because I want to play with it.