paphs and phrags make a ton of leaves per growth, if you count the small ones at the start of the growth. it is normal for them to yellow throughout the year, although around this time of year you stand to see more of them fall off. as long as they're the oldest leaves, it's probably nothing to worry about. sometimes old growths will die off too. the really healthy plants with tons of roots tend to keep the old growths around for longer, but some plants just don't maintain more than a growth or two at a time.