Why do shore power neutrals get too hot?

Looking at the photo above which is showing the terminal on a shore power neutral conductor that has gotten a bit too warm reminded me of a question posed to me earlier this at the IBEX show in Louisville. One of my ABYC certified technicians asked me why it seemed that it was almost always the neutral conductor that got burned or melted when there was a problem with shore power. The answer is not simple. Now, in the above photo, the cause of the heat could have been simply a loose screw ...