Found as close to an official take on auto-rout (and dismayed by destruction of army) while looking at a completely different thread that meandered off:
longjohn2
Programmer
Group: CA
Posts: 263
Joined: Nov. 2000
UK
Posted: June 26 2003,17:02

-----------------------------------------------------------
/snipped

The army destroyed penalty kicks in when most of an army's combat power has been eliminated. Again it's intent is to speed up the end of games by getting rid of small units that have rallied well away from the action, and would be a chore to hunt down.

The disheartened by constant retreat penalty is related to the Benny Hill detection code that routs your unit if you try to win a battle by running around avoiding combat until the timer runs out. As the retreat counter builds up you get a morale penalty that serves as a warning, and may if you're lucky tip your unit into a normal recoverable rout, rather than the unrecoverable Benny Hill penalty rout

/end