Virgil is imitating a Greek construction here, or rather two Greek constructions: the middle voice and the accusative of respect.
Greek had a "middle" voice, which in most tenses was formally identical with the passive, but did not have passive meaning; rather, it expressed actions in which the doer was also somehow affected by the action. It's hard to define the function of the middle voice precisely because it had a very wide range of meanings, but it could have been used in the context of this line, and Virgil is imitating that usage by using the passive, which is the closest thing Latin has to a middle. It's not too far from an English reflexive, "having pressed herself...".
Greek also had a particular usage of the accusative called the "accusative of respect". This can be translated as "with respect to...", and it's often used with words for body parts: for example, in Homer Achilles is often called "swift of foot", but the word for "foot" is actually in the accusative, "swift (with respect to) his feet".
So putting the two together you might very literally translate, "having pressed herself with respect to her face...". Virgil uses this construction (or combination of constructions) a lot: cf. e.g. 1.229 oculos suffusa nitentis, and many other examples.