How did Norway manage to attack Scotland?

The Scottish-Norwegian War of the late thirteenth century began due to the belligerence of the Scots in demanding the return of the Outer Hebrides and the Isle of Man, which they'd previously ceded to the Norwegians. The Scots were prepared to buy back the islands, but the Norwegians wouldn't negotiate, so the Scots set out to take them by force.
The Norwegians responded to this threat by assembling a large fleet of warships. The Norwegians were expert mariners (just like their Viking ancestors), and this was their preferred method of waging war. However, things went wrong for the Norwegians; their fleet became stranded in stormy weather off the west coast of Scotland. An indecisive battle ensued, after which the Norwegians headed north to the Orkneys to spend the winter.