In the Middle Ages, why were Frenchmen referred to as "crapauds" (toads")?

Crapauds or Johnny Crapaud. A Frenchman; so called from the device of the ancient kings of France, "three toads erect, saltant." (Guillim's Display of Heraldrie, 1611.) Nostradamus, in the sixteenth century, called the French "crapauds."

http://www.infoplease.com/dictionary/brewers/crapaud.html
https://en.wikipedia.org/wiki/Crapaud

The usage of the word "crapaud" in this case is similar to the derogative use of the word "frog," referencing the supposed French affinity for frog legs as a delicacy, and thus considered to be a slur and derogative.

https://en.wikipedia.org/wiki/Crapaud

Crapaud French Originally the name of a South European frog, although during the Napoleonic Wars it became a term used by British soldiers referring to their French enemy.

http://www.rsdb.org/search/french