Hi there!

What you are referring to here is the rout, or disordered flight of an army. It could often be the bloodiest part of a battle during the Medieval period.

A rout was usually started through panic. Maybe your general has fallen, or your rear is exposed, or your lines are fragmented. Maybe a rumour of defeat or loss spreads among the troops. In any case, this triggers a retreat in a section of the army, which grows exponentially until an entire force is fleeing. Keep in mind that not all retreats are routs, some are very effective and orderly, even brilliant, but the rout is chaotic.

Now two things happen. First, the victorious side has the field. In many examples, this means that they can capture everything the fleeing side has left behind in their haste. Some of this we call baggage; the valuable materials of the army and things their enemies can subsequently use against them or sell or keep as trophies. Other materials will likely include the artillery or engines of the army, things too heavy and cumbersome to flee with: they now also become the property of the victor.

Second, begins the carnage of the rout. Horses flee faster than men. Light archers flee faster than mailed knights. The routing force is quickly disjointed and separated in this way. You can scarecely defend yourself from behind while running away. So it is that the fleeing army is cut down as they are pursued, often losing many more soldiers in retreat than they did in battle.

Overall, the rout was a costly failure for an army. The losses in manpower, materials and prestige could be catastrophic, compared to those of an army which successfully retreats in good order. If the Medieval period interests you, the more you read about campaigns and battles, the more you will recognise the telltale hallmarks of rout above. A good place to start would The Hundred Years War. You can probably identify in the battles at Crécy and Poitiers the kind of panic, retreat and losses that are described above.