I can't be sure if there is a specific reason relating to Scotland, but I was told when raising a similar query on English ancestors that it is akin to the use of double-barrelled names today. I think it can sometimes be linked to illegitimacy, but I have find several generation of one family use "Reason alias Hucks" as there family name. One suggestion for this was that the family wanted an earlier connection to an earlier relationship between two separate families to remain, possibly relating to inheritance factors.

(Apologies for grammatical errors, etc., in my previous post.)

A very useful link provided by @JBPurdie. Deals with the issue comprehensively.