The great divide in Scotch aging is between ex-Bourbon barrels and ex-Sherry casks. Up until 1946, most aging was done in ex-Sherry casks because British shippers imported Sherry into the country in cask and would sell the used casks to Scotch distillers dirt cheap once they had bottled the contents. But in 1946, US law began to require that Bourbon be aged in new, charred oak barrels. This meant that Bourbon distillers could only use their barrels once and were more than willing to sell them cheap to anyone who would take them. That, combined with Britain's steadily shrinking Sherry consumption, caused most distilleries to change their aging from virtually all ex-Sherry casks to virtually all ex-Bourbon casks. Most distilleries had some of both, of course, and the finished product would have been composed of whisky from both sources. But until Glenmorangie decided to try it, no distillery considered racking aging whisky from Bourbon barrels into Sherry casks or vice versa. It was a fantastic success, and so now just about every distillery copies them. And not just with Sherry. Distilleries have used Burgundy, Madeira, Sauternes, rum, and many other kinds of casks and barrels to "finish" their Scotch. Including Port. Port makes a lot of sense. Back in the 19th and early 20th Centuries, Britons probably drank as much port as they did Sherry, so lots of ex-Port pipes must have been used to age Scotch. So, given where Scotch had come from and the innovation that Glenmorangie made, it is perfectly reasonable that Diageo would have chosen to finish their Cragganmore Distiller's Edition malt in ex-Port pipes.
Port offers many of the same characteristics that Sherry does to Scotch, especially color and sweetness. Where Sherry adds nuttiness, Port adds fruitiness. I've never tried Glenmorangie's Port-finished malt, but I have a hard time believing that it's any better than this. This wood-finishing business seems gimmicky and largely marketing-driven to me, but it sure makes for delectable whisky.