I had an anterior placenta with both boys, and neither was posterior during labour.
With DS2 i was told throughout the pg that he was in a posteroir lie, but he wasnt when labour came around.

It is supposedly more likely as they liek to face the placenta, but also my midwife told me that if its not your first pg, and especially if you had a bigger bub previously, its not uncommon for them to be posterior during pg.