Ben Witherington suggests that the final few lines of the Mark autograph1 were worn away thusly:
If the Mark scroll were rolled with the beginning on the inside and the end was usually exposed, it's possible a portion of the papyrus and the ending would be lost. Rather than losing an entire column, only a few verses would be missing. Dr. Witherington suggests the ending was available to Matthew and postulates:
And behold, Jesus met them and said, “Greetings!” And they came up and took hold of his feet and worshiped him. Then Jesus said to them, “Do not be afraid; go and tell my brothers to go to Galilee, and there they will see me.”—Matthew 28:9-10 (ESV)
What makes this passage so interesting is that Matthew was quoting the final lines of Mark at this point2:
And he said to them, “Do not be alarmed. You seek Jesus of Nazareth, who was crucified. He has risen; he is not here. See the place where they laid him. But go, tell his disciples and Peter that he is going before you to Galilee. There you will see him, just as he told you.” And they went out and fled from the tomb, for trembling and astonishment had seized them, and they said nothing to anyone, for they were afraid.
(Mark 16:6-8 ESV)
But the angel said to the women, “Do not be afraid, for I know that you seek Jesus who was crucified. He is not here, for he has risen, as he said. Come, see the place where he lay. Then go quickly and tell his disciples that he has risen from the dead, and behold, he is going before you to Galilee; there you will see him. See, I have told you.” So they departed quickly from the tomb with fear and great joy, and ran to tell his disciples.—Matthew 28:5-8 (ESV)
Then, after Matthew used the Mark autograph as a source but before it was copied verbatim, the ending was lost. Luke tells a slightly different Easter story. Later, when 4th century scribes noticed that Mark ended awkwardly, they "reconstructed" Mark 16:9ff from resurrection appearances in Luke and some details from the Acts of the Apostles. Later scribes who had access to copies either with or without the "long ending" naturally chose the more satisfying conclusion.
Having summarized the "wear and tear" theory, I need to mention a few points against it:
In order for Matthew to have the original ending, he would have had to be working from the autograph itself.3 While not a damning conclusion, it does seem to be a bit coincidental.
In order for the ending to be worn away, many years would have necessarily passed before the first copy of the text were made. Even after Matthew used the master as a source, enough time would have had to have passed for the scribes to have forgotten the connection. Otherwise they would have used Matthew to retrieve the ending.
The owner(s) of the master scroll would have had to neglect it enough to not notice the ending rubbing away, but not so much that they wouldn't make a copy. That's a very specific level of neglect. If there were a series of owners, the ones most likely to know that the scroll was valuable (the earliest) would have been least careful about preserving it while later owners (to whom it must have appeared to be summary of Matthew) spent the money and time to copy it.
Matthew completely changes the emotion experienced by the women in anticipation of the resurrection experience he records. If he were working from Mark, the impact of Jesus Himself showing up in Matthew 28:9 would have been blunted by allowing women to experience joy rather than just fear. To me, it makes more sense for Mark to emphasize the fear and timidity of the women because he didn't ever plan on including a resurrection appearance.4
There are any number of dangers an ancient manuscript might suffer to cause Mark's original ending to be lost. But the story is a bit more complicated in the case of a scroll format that Mark originally must have circulated in as compared to the codex format.
1. Or just the "master copy" to which all manuscripts of Mark must trace their ancestory.
2. I've bolded words that Matthew seems to have taken from Mark and italicized places where Matthew seems to diverge from Mark. Some words are in both categories.
3. The idea paints a very interesting story of the author of Matthew obtaining Mark and instead of making a verbatim copy, going over it to augment and correct the first draft of the gospel. Then, since the first draft was not destroyed, both versions eventually saw circulation. That would fit well with the concept that the early fathers had that Mark was later than Matthew, since from their perspective it popped up later.
4. I subscribe to the logic of the NET Bible footnote on the end of Mark.