My dicentra exima leaves are yellowing where they are crowded with others-leaves touching.

Maybe overwatering. They require good drainage and are fairly drought tolerant. They sometimes will go dormant from lack of moisture, but will return with new growth when the soil becomes moist again.
Once established and the roots spread out in the surrounding soil, they are tough little natives!

Thanks for the responses! Yes, I am going to put the responsibility on the over-generous amount of rain; the plant certainly isn't crowded. I'll be certain to lighten the soil more before replanting.

I bought the plant this spring from White Flower Farm and said "Wow!" at the generous size of the plant that I received. It was definitely a top-size and top-quality plant. Fortunately (in hindsight), I wasn't able to resist dividing it before planting. The other plant, several feet away, seems to be doing fine, so it wasn't a total loss.