[Zoo]

The uterus doesn't contract evenly, IYKWIM? When the last contraction births the baby, majority is from the sides of the uterus. The placenta doesn't fill the uterus like the baby does and therefore the uneven contraction can break away only parts of the placenta thus causing it to go into pieces and leave retained products behind.

Like I said, it happens and then it doesn't happens. Studies are showing that retained products are now having a link with the duration of labours aswell.