Well, I can think of a couple of reasons that are not exactly "laziness." I have heard of people doing this when they wanted to name after two people who the same first initial, for instance Louise and Lawrence. They then give their child the middle name "L." to honor both individuals.
Also, parents from some cultures where middle names don't exist may not really understand them. They may see someone sign his name as John Q. Jones and think the Q itself is the middle name. For example, I know of a Japanese boy who was born in the U.S. to parents who were here temporarily. (In Japan, everyone has only a first and last name, there are no middles.) These parents wanted to do something special that would commemorate the fact that the boy was born in the U.S., so they gave him a middle name-- "C." It is just C. on his U.S. birth certificate, and doesn't appear on his Japanese birth certificate at all. When others asked what C. stood for, they thought a minute and said it was for C-section, and really thought it was quite a funny joke.
I don't know why your mother just had the middle name K. Did she ever ask her parents?