M. Eugene Boring’s discussion of Jesus’ transfiguration event in his commentary on Mark, begins by suggesting that, “On the way to its incorporation in Mark, several influences may have played a role in the formation and transmission of this story in which the human Jesus not only converses with heavenly beings but is himself clothed with an otherworldly glory.” Apparently by “formation” Boring believes that Mark’s account has been so influenced by other similar accounts in such as Ovid’s Metamorphosis and 1 Enoch that, “It is thus difficult to regard the account as simply a straightforward reporting of an event in the life of Jesus.” Boring goes on to reject subjective interpretations (i.e. “a group visionary experience”), rationalistic explanations (i.e. natural phenomena [lightning, thunderstorm, fog, sunset or sunrise glow]), and literary-theological explanations involving “a misplaced resurrection account.” Boring suggests that these explanations have “failed to meet the test of scholarly scrutiny.” Rather according to Boring, “While the transfiguration is not a story of a specific resurrection appearance retrojected into the pre-Easter life of Jesus, the Markan narrative as a whole is indeed seen from the perspective of the risen Lord of the church’s faith, so that there is a sense in which much of his narrative is a retrojection of post-Easter faith onto a pre-Easter screen. In early Christian theology in general and in the Gospels' narrative theology in particular, the line between historical Jesus and risen Lord was not firm or crisp.”
M. Eugene Boring, Mark, Interpretation (Louisville: Westminster John Knox, 2006), 260–1.
To be honest, I fail to see why it is so hard to accept Mark’s account as reflecting an event that actually happened in Jesus’ life. I am unpersuaded by the suggestion that Mark must have been so influenced by either similarities to other literary accounts or by a post-Easter faith to the extent that his recording of the Transfiguration account involves either fabrication or retrojection.