my guess is you over-fed and burned them. It is hard to over-water hanging pots. I would flush more with water and hope for the best.

Were the baskets on clearance? Three weeks ago is too late to charge full price for a basket of petunias, assuming they've been hanging around since spring. Don't recall ever seeing a fall shipment of petunias, but not looking either. Do "they" do that?

I disagree about the ease of overwatering the hanging baskets *if* the basket is the type with a recessed drain hole about 1/2 inch above the bottom surface of the pot. That allows 1/2 in of standing water in the pot. If it was fertilizer water, that could have been even more overwhelming than when it is plain water.

It's hard to find the right amount of sun for some plants in hanging baskets. The hook is usually hanging from something, casting a shadow - OR - sticking out so far there's never a moment of shade.