Sunburn would be more evenly distributed over all the leaves. It is not normal die back of older leaves either nor is it lack of nutrients. Most likely it was allowed to get too dry at some point and perked up after getting water. However, selective leaf yellowing often occurs several days to a week after a drought episode. That said, there is no long term damage to the plant. Trim off the yellow leaves as recommended here and avoid allowing it to reach the wilt point again. Peace lilies don't tolerate dry soil. Also avoid any direct sun rays falling on the leaves. To promote flowers, keep it potbound and fertilize it at half strength monthly.