In World War I, what did "mud cruncher" refer to?

During World War I, infantryman were given many nicknames to denote their lowly status. These nicknames included footslogger, ground pounder, rat, and gravel crusher. A favorite nickname for soldiers during World War I was the mud cruncher. Being an infantryman during World War I was a thankless job. You spent your time in the trenches for months on end in miserable conditions. Your life was in constant jeopardy from mortar attacks. As an infantryman, you were expendable.

The moniker is the name of a book by Matt Riddle that chronicles the experiences of a Carolina farm boy and his experiences in the trenches. The genesis of the nickname mud cruncher is uncertain, but it probably refers to dead bodies in the trenches with their bones being crunched by tanks.