We see this rule all the time in other systems - the bad guys are outnumbered, their leader has been taken out, most of the group has taken damage and it looks like its going to be a rout - roll vs X or scatter and retreat!!!
How is that simulated in the context of FATE? Does a player make a declaration? Does the GM plunk a new aspect on the scene? Is it just "common sense" and lo, therein goes the rascals, hither thee go?