That's a big if ... I'm not sure that the author of Mark was 'there'.I really do think that this incident was so traumatic and experience for the author that he made mention of it. If it had happened to you I think that it would have been sharp in your memory as well.
Actually I do, and there has been a growing number of scholars who do, too.Surely you do not think that this gospel was written by an experienced story-telling literary author?
In their view, Mark is writing to a popular audience, in a popular style, and very successfully, whereas Luke, for example, is writing more for the literati, an educated audience.
I mean, Dan Brown's English is execrable, but he's certainly popular.
Well there's no actual evidence that it was exactly 'this'.Because it happened, a memory so traumatic that the author made mention of it. If it had not been exactly this then your 'why?' question would be much stronger.
On the other hand, if the scholars are right and Mark is writing a fast-paced and dynamic tale to be 'performed', then such a device would be very useful – not the least to imply the author was 'there' and add another layer of drama.