I recently watched a documentary on Robert the Bruce and it made a statement something like, "Only clan MacDuff had the right to crown the king of Scotland and would have to do so again if there were a current king of Scotland"; or something to that effect. I've tried researching this but all the articles only say that it had to be a MacDuff which crowned the king but none of them state why.
The lands that MacDuff held, had the lands of the Church of the Scottish monastery at Abernethy. Along with this came the title of Lord of Abernethy
The church had the Stone of Scone (The Stone of Destiny), which Scottish Kings had been crowd upon for centuries.
The chiefs of the clan had the right to enthrone the King on the Stone of Scone.
Sorry I'm a bit late replying to this; end of semester is always a bit mad for a grad student! The answer to your question lies in the 'Privilege of MacDuff', which can be traced back to the reign of Malcolm III of Scotland (1058-1093). In light of great services to the crown by the then lord of Fife, Duncan MacDuff, the MacDuffs were granted the following privileges in perpetuity: 1. That Duncan, and his successors lords of Fife, should have the right of placing the Kings of Scotland on the throne at their coronation; 2. That they should lead the van of the Scottish armies whenever the royal banner was displayed ; 3. That if he, or any of his kindred, to the ninth degree, committed slaughter of a sudden (i.e., not pre-meditated) nature, they should have a peculiar sanctuary, girth, or asylum, and obtain remission on payment of an atonement of money. These rights were said to have been granted to the MacDuffs at Malcolm's first Parliament in Forfar.
Charles Ferguson, 'Sketches of the Early History, Legends, and Traditions of Strathardie (No. V)', The Transactions of the Gaelic Society of Inverness, 21 (1896-97), p. 74.
John M. Leighton, Joseph Swan, and James Stewart, History of the County of Fife from the Earliest Period to the Present Time (Glasgow, 1840), p. 24.