Are you aware that his service record has survived and can be found on FindMyPast (and probably also on Ancestry)? Unfortunately there is no copy of his MM citation among his documents, although there are his signed receipts for it and his other medals, the 14-15 Star, BWM and VM. As LittleNell says your best bet for finding out why he got the MM is search the Battalion war diaries. You will find them at:
WO95/1941/1. Fortunately they have been photographed so once your have registered with TNA, you can download them for free.
He enlisted on 27 Oct 1914 at Glasgow. He served in C Company, 7th Battalion Cameron Highlanders. On 27 July 1918 he changed his name to Thomas Alexander McMurray. I think the reason he did this is obvious.