I was told that Scottish mouth music came about because of the English ban on the the traditional instruments of the Scottish people. The Scots at the time simply transformed the sounds of pipes and other instruments into vocal sounds and phrases.

Interestingly enough, French Canadians also have a strong tradition of mouth music. I was informed by a Francophone from Quebec that French mouth music was created for the same reason as Scottish mouth music. The English wanted to control the Catholic french people of early Canada by banning their music & religious practices. The French simply turned to mouth music as an alternative.