Use of the passive in Caesar "agros populabantur" to indicate state of action

Question

At first there seemed to me to be a grammar error in De Bello Gallico I.11:

Helvetii iam per angustias et fines Sequanorum suas copias traduxerant et in Aeduorum fines pervenerant eorumque agros populabantur.

Because the Helveti are the subject and they are not being devastated, the fields are. So, I would have expected either:

eorumque agros populabant ("they were devastating their fields")

or

eorumque agri populabantur ("their fields were being devastated")

However, on consideration and after consulting translations it seems like the passive tense is being used here to indicate not that the subject is being acted on, but that the subject is in a state of acting a certain way. So, in other words the phrase means "[the Helveti] were engaged in devastating their lands", as though it said eorumque agros populanti or something like that.

Is there a term for this use of the passive?

Answer 1

As Sebastian noted, populor is a deponent verb: it has a passive form but an active meaning. Deponent verbs can take direct objects in the accusative, so the translation is indeed:

they were devastating their fields

populor is somewhat unique because the corresponding non-deponent form populo is also often used.