According to Metzger, "The longer ending..., though current in a variety of witnesses, some of them ancient, just also be judged by intrnal evidence to be secondary" (A Textual Commentary on the NT, 2nd ed., p. 104). Why? "The vocabulary and style of verses 9-20 are non-Markan" (Ibid.). Then he goes on to give some Greek words that occur in the passage but not in the rest of Mark. What is wrong with this picture? It's actually not that rare for such a thing to happen.
It is said that vv. 9-20 guilty of violating the style of the rest of the book. In what ways? I know this is thrown around quite a bit, but I rarely see attempts to prove it. My contention is that with only one book by Mark, it is presumptuous to say we know his style perfectly. Also, consider this. Phil. 2:5-11 (a shorter passage than the Mark ending) has four words found nowhere else in Paul's extensive works: "robbery," "equal," "highly exalt" (one word in the Gr.) and "things under the earth" (one word in the Gr.). Plus, the style is unique! Nowhere else to my knowledge does Paul use this poetic style. So, do we throw out this passage? No, of course not. No scholar does! But the same arguments could be made against it that are made against Mark's longer ending. There are other similar passages in other books, too.
I had a short book published in 1979, The Making of a Soul Winner (I'll attach a PDF here for your enjoyment). Check out the chapters. The first two are anecdotal, the third is more hortatory, and the 4th is didactic. Because of their different purposes, the vocabulary can be quite different from chapter to chapter. So, I reject the view from internal evidence that the longer ending of Mark is bogus because of vocabulary differences.