Speaking from general rose experience, not from knockout rose experience, and my rose care is pretty lackadaisical, but ...
Let me suggest a sequence, almost a flow chart. This may NOT be what has happened, and is completely DIFFERENT from what the previous respondents have suggested.
1) You were dead-heading the roses "way back when," when your neighbor suggested the hedge trimmers. Had you pruned them last fall or in the spring?
2a) If you did prune them last fall or in the spring, I believe the center turned brown due to excessive heat and lack of water. The outer leaves can glean some moisture from overnight condensation, even the small amounts available in most parts of the country this summer, but the inner branches? No. (Note: I don't know *where* in North Carolina you live; maybe you've had rain.)
2b) If you did not prune the roses within the last year and they were somewhat overgrown when you went at them with the hedge clippers, there's a possibility that the center not only was dried out, but that it has *also* suffered from sunburn (called "sun scald" when it happens to plants, but we all understand what it means) because of the sudden exposure to full-on sun in midsummer in the South. Ouch. That, plus the many temperature records set this year in our country and no water...well...see 2a).
3) Insects can be vectors for disease. I don't know which disease(s) the Japanese Beetles specifically can transmit, but they cannot have been good for the plant. They may have weakened it so that, when the hedge clipper episode took place, the "browning out" followed.
My recommendations--I always err on the conservative side, leaving room to take more drastic action later, if necessary. It's difficult to replace foliage or branches that have been removed later, but always possible to take them off at a later time.
1) Use a hand fork to lightly scratch in some compost near the roots of the rose bushes. It's fine if most of the compost remains on the surface; watering will move some of it down, and earthworms will also bring a lot of it down to the root zone.
2a) Then, unless you're on water restriction, give the roses a DEEP watering. Use a trowel to dig a 3- or 4-inch deep moat around each bush at its dripline, approx. 2 to 3 inches wide. (This will also move some of the compost beneath the surface.) Fill this moat with water. Let the water drain to the roots of the bush. Fill it again. Let it drain again. Repeat in a week. Do this as long as there is no decent rain ("decent rain" = > 0.5" in one event).
2b) If you *are* on water restriction, take a 3-gallon bucket or convenient sized pail into the house. When you're waiting for the shower to warm up, let that water run into the bucket. Later (e.g., after the shower!), pour the water into the moat. If you have a laundry sink that your washer drains into rather than hidden plumbing, use the last rinse water for the roses. Again, catch the rinse water in the bucket (you may need to stop the washer a few times to catch all the rinse water) and transfer it to the roses. A top-loading washer should take care of the roses nicely with just one or two loads (I have a top-loading washer and yes, I have transferred rinse water to my plants previously).
Sunset Zone 17, USDA Zone 9