Yes it is normal for most beans and at the density shown in your pictures. Lower leaves provide little photosynthetic benefit to the plant, thus the plant mobilizes nutrients from lower leaves and casts them....just about all plants do this 'catabolism' as maturity advances....grasses start at about 5 leaf stage as example. Old leaves are 'retired'.....even on healthy soil...but may not be apparent on very high fertility, irrigated and well drained soil. Leaf cast is most evident in beans when pods develop as the leaf serves as a 'nutrient reserve' for the seeds in the pods! Extreme weather conditions can promote leaf casting as well. Plants look healthy to me....some minor rust....some minor soil pH issues! When was the last liming and potash application?