Soybean pod shattering is not completely understood. Through breeding efforts (beginning approximately 5,000 years ago when soybean was first thought to be domesticated), soybean plants have gained pod shattering resistance (Dong et al., 2014). However, certain environmental conditions can lead to pod shattering.
Last week, I received a picture of empty, curled soybean pods which is an indication of shattering loss (top picture). This particular field received hail which could explain the empty pods. With the recent wet weather, shattering can also occur as a result of the re-wetting of dry pods. We saw soybean pods shatter in 2012 when pods were formed under drought conditions and re-wet later in the season.
Shattering (and other harvest losses) can account for significant yield loss. In the bottom picture, there are approximately 8 to 12 seeds per square foot which translates into a 2 to 3 bushel per acre yield loss.
Reference: Dong, Y., X. Yang, J. Liu, and Y. Wang. 2014. Pod shattering resistance associated with domestication is mediated by a NAC gene in soybean. Nature Communications 5:1-11.