Dr Erica McAlister, senior curator of Diptera at the NHM, replies:

Flies have amazing vision and are able to process and act upon external cues rapidly, such as you or another predator. Predators trigger an escape response, causing the fly to jump out of the way and then fly away but, in your case, the fly must have weighed up its options and decided that hiding was the preferred option.

Hoverflies are capable of hiding from predators.