I have no idea but I have the same situation as you - DD1 and DS were both anterior until the second that I went into official labour and they both turned posterior. Ouch! DD2 decided to be breech but my labour was so completely different with her (after over 40 hours with DD1, I was fully dilated after 40 mins with DD2 and it didn't hurt anywhere near as much - DD2 was born by emergency c-sect though as they wouldn't let me give birth naturally due to her position).

I think the official position on having OP babies is that there is no reason for it medically, but a midwife suggested to me this time, there must be something about the shape and position of my uterus that means all my babies want to be in odd positions for birth. I don't think anything is proven though and I don't know if they can tell.

After DD1, I thought it was something about me and the things I was doing that made her OP so I spent a lot of time with my head down and bum in the air in the lead up to labour with DS, I spent very little time sitting or lounging on soft chairs and couches and did lots of walking. None of it made any difference.

Sorry I couldn't be more useful in my answer, but I thought it might help you to know that other people are in the same boat.