Why is nominative instead of ablative absolute used in 'Ibi egressi Trojani'?

Question

In LLPSI 2 'Roma Æterna', Chapter XLI 'Origines', it is written:

Ibi [Siciliâ] egressi Trojani, quibus ab immenso prope errore nihil præter arma et naves supererat, cum prædam ex agris agerent, Latinus rex Aboriginesque, qui tùm ea tenebant loca, ad arcendam vim advenarum armati ex urbe atque agris concurrunt.

I was wondering why egressi Tojani is used here instead of an ablative absolute egressis Trojanis, just as in mortuo Cæsare, which is a common example with another deponent verb. In LLPSI 1 there is an example of Hæc locuta, which is stated to mean the same as Postquàm hæc locuta est. However, I still do not understand why the ablative absolute is not used instead: Hæc locutâ (long â, ablative).

I have another question regarding this passage. Why is the verb agere used and not facere in the sense of make a bounty of: cum prædam ex agris agerent and not cum prædam ex agris facerent. I often have trouble mixing up these two verbs. Is there a rule of thumb to know when to use either?

Answer 1

Egressi Trojani is in the nominative because it's the subject of agerent. The structure of the sentence is a bit unusual, but it's clearer when you move the cum to its vanilla position before the egressi Trojani, since the whole thing is a subordinate cum causalis:

Ibi, [cum egressi Trojani, quibus ab immenso prope errore nihil praeter arma et naves supererat, praedam ex agris agerent,] Latinus rex Aboriginesque, qui tum ea tenebant loca, ad arcendam vim advenarum armati ex urbe atque agris concurrunt.

Translated:

There, since the disembarked Trojans, to whom due to the almost immense wandering nothing except weapons and boats remained, were carrying plunder out of the fields, the Latin king and the natives, who then held those places, armed to defend against the attack of the foreigners, ran together from the city and the fields.

Ago can also mean 'to steal, to rob', and praedam agere is a fairly common idiom.

(As a side note, this passage is from Livy's Ab urbe condita, though it seems to be slightly modified: Livy has superesset instead of supererat.)