Answer: This is really puzzling. It may take some detective work to figure it out.
Since there is a progression from hydrangea to hydrangea, I would look carefully at the environment around them. For example, if you use an automatic watering system, is it working correctly? If you use a soaker hose, is that end receiving less water?
If there is a walk nearby where you use de-icing salts, are they washing into the bed -- more so by the more wilted one? If you use a weed and feed product on the lawn, did it accidently contact the plants -- more so by the worst wilted one?
I would also look to surrounding trees -- some have very greedy roots and could be rooting out into the planting bed and stealing moisture from it. (This could account for the progression in the hydrangeas and also for the petunias seemingly being random.)
Also look carefully for root damage, voles for instance will eat and damage roots.
Is there a black walnut tree nearby so that the plants could have come into contact with juglone from roots, leaves, stems or nuts? Via the mulch?
Have used straw in your compost? There are some herbicides used in crop production that could potentially carry in on the straw and affect your plants.
Gas leak in an underground pipeline to the house?
Petunias are subject to some diseases, you might want to have your county extension take a look at that, in case there is a disease problem. (Some can be carried on tools or soil.)
Dig up a couple of the affected ones and examine the roots very carefully. Can you see anything unusual?
HAve they rooted fully into the surrounding soil? If you planted rootbound plants and did not cut or unwind the roots at planting time, the roots will continue to grow in a constricted pattern. This will eventually strangle the plant. With shrubs, it can take several years, but I have even seen it take seven years for the plant to finally die -- I mention this because it could be a matter of slightly different watering combined with this kind of root stress.
In my experience there is always an explanation when something like this happens, it may just require some out of the box thinking to figure it out. Good luck with the mystery!
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