Pt. Reye's Ceonothus

Fairly new to NG's hope you do not mind my posting a query. Moved into this newl constructed house 10 yrs ago. Only the front yard had a bit of grass. Planted a number of Ceonothus Pt. Reyes as a ground cover. The lot is a half acre so did not want much mowing. The ceonothus finally took over an area of about 20 X 40. THis year many of them are dying out. Or appear to be burned. No idea what is causing this. Also, this area is over the septic drainage field. Although its a prostrate variety was shocked to see how far some of the roots have gone and how thick they are. Any suggestions? Should I have kept them clipped to a more manageable size? Any info would be greatly appreciated.
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