Fighting spread out in front and in small groups, the light infantry were easy picking for lightning fast cavalry. This didn't change through out history in fact and was most noticeable in the Napoleonic wars.
Here, realizing he has been cut off, a Centurion barks the order to fall back, it will be the last he ever gives. The long slashing Spathas of the cavalry make short work of the tiny struggling knot of Auxiliaries. Way back in the formations, other men look on, powerless to help their beleaguered colleagues..