Well the 'retreat-path' isn't neccesarily leading back to from whence they came; it may actually (I have observed this rather 'odd' behaviour myself on numerous occasions) lead further into your territory or be about some very elaborate grand manoeuvre... The AI is crazy like that.
As for your story of the entire army crashing through your lines -- that's the usual stuff when the retreat route would lead 'em accross the bridge.Happens elsewhere too... the bridge a Ptolemais-Theron, some of the river crossings in Delta-Neilou. I guess it just happens if the AI has a bad day and makes the wrong decision about army-movements on the strat-map.
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