It is of course disappointing even if you find your man's name to find no reason behind the award of an MiD.
This example page from a 1945 London Gazette
https://www.thegazette.co.uk/London/issue/36979/supplement/1385illustrates that in many cases the "mention" did not result from one specific act but was for sustained activity over a period thought worthy of recognition just short of a medal. Note particularly the MiD to Lt Cdr Richards covering his duty over an almost 6 month period.
In very crude terms, the process within the chain of command was for recommendations to be submitted up the chain, not all of which resulted in an award and often in response to a request from the top level for names to be passed upwards. The awards on the left for the anti U Boat operations demonstrate clearly the "hierarchy" of awards. In one sense, the fact of the publication of the "mention" should be seen as the award.
Now and again a unit war diary or squadron operations book would hold a clue, (although not necessarily the name) in descriptions of operations of the period. Linking with other men who received higher level awards at the same time sometimes gives an idea. (In lordcobb's case, I do hope the documents cited by the ever informative Seaweed have something).
None of the foregoing is to take away anything from the devotion to duty of men MiD but to attempt to explain why the search for a reason in military documentation more often than not will yield no result.
MaxD