I'm a far cry from expert or anything... but sounds to me like hitting it with a fungicide might be a good idea. The pattern you describe sounds to me like disease. A spray bottle of Imunox or similar is not expensive, might help, won't hurt.

Another possibility could be something eating the roots.

i saw some sweet woodruff at a garden last year which looked to be in a similar state of yours, and i do believe it was cause by excessive water coupled with inadequate drainage. Could you move these plants possibly to higher ground or amend the soil to increase your drainage? this might help.

I've got sweet woodruff contained by my driveway on one edge and you're describing how it looks after it's been hot enough for the driveway itself to cook the plants. Is it possible that your rocks have gotten hot enough during the day to keep the plants near them particularly hot? Could there have been some sort of foreign liquid in with your daily rain that washed onto these plants? (animal urine in high concentration, cleaning product from the bricks, etc.) If the plants away from the rocks are okay it's probably not excessive water, unless these are the "downhill" plants that would get the bulk of the water.

Thanks for the input. The plants seem to have stopped dying off and there is some new growth where they died back. I don't know if it was too much rain; the rain has not been nearly as frequent. The area where they are planted does not get hot enough to kill them and the drainage is too good--they are planted under giant white pines that suck up the majority of the water. I haven't used any cleaning solutions on the bricks. Hopefully, the dieback has stopped and the plants will come back next year ok. Thanks again.