The reason Jerome was in Bethlehem was essentially because Rome was no longer a safe place for him to be in: his patron Pope Damasus had died, and he was very unpopular among the Christians (especially the priests) in Rome, because he was very harsh in his condemnations of what he saw as decadent corruption on the part of the secular clergy, while being surrounded by a circle of women followers, from well-born families. (In other words, Jerome was attacked as a hypocrite.)
One of Jerome’s closest followers was Paula, a wealthy widow who hailed from one of the richest senatorial families in Rome. Under Jerome’s influence she and her daughters (Eustochium and Blaesilla) gave up their wealthy lifestyle, but the rigors of asceticism took a toll on Blaesilla’s body that she died soon. That was the last straw for the Roman Christians: they blamed Jerome for killing Blaesilla. (It did not help that Jerome tried to dissuade Paula from grieving over her daughter.) So it was in that context that Jerome, along with some friends of his, left Rome for good to settle in the Holy Land. Paula and Eustochium soon joined him there.
And the translations from the OT he made while he was in Bethlehem was unlike the revision of the NT and the Psalms that Pope Damasus ordered him to do years back, in that it was not done under official pressure. It was just a private project (just one of the projects he was doing at that time, in fact - the man was a workaholic), which he did because his friends kept asking him to translate books of the Old Testament from the Hebrew. And he was doing this while writing other books and being involved in the (very messy) Origenist controversy instigated by another saint - Epiphanius of Salamis!