Why does turbofan exhaust appear to reverse direction?
What you are describing is a characteristic of fluid dynamics known as eddies. As different portions of the fluid flow faster than others, the friction and surface adhesion between the two layers cause swirling of the fluid and reversal of the current, creating a turbulent flow region. It is this same principle that holds a ping pong ball stationary in a moving column of air.
For a simple overview, you can check out this wikipedia page:
https://en.wikipedia.org/wiki/Eddy_(fluid_dynamics)
For a more scholarly exposition, try this:
https://arc.aiaa.org/doi/abs/10.2514/2.1794?journalCode=aiaaj