In the most recent issue of Theology, Philip Oakeshott suggests his own solution to the unease raised by the abrupt εφοβουντο γαρ as the ending of Mark. He suggests that only the first part of the sentence has survived and that it must be amended along the lines of Mark 6:49-50a. There the disciples are terrified seeing Jesus walking on water and think that he is a ghost, a φαντασμα. The last sentence of the gospel might have read, according to Oakeshott, something like εφοβουντο γαρ, δοκουσαι οτι φαντασμα εστιν. Since the early church, as already seen in Matthew and Luke's reworking of Mark, did not think it the best of ideas to refer to the resurrection as seeing a ghost in the shadow of a tomb, the ending did not last very long.
Oakeshott, Philip. "The Wrong Conclusion: Mark 16.1-8 and Literary Theory." Theology 113, no. 872 (2010): 105-13.