Why do wave petunias start to seed part way through the summer? They looked good for June and most of July, then the flowers became sparse and started to produce seeds. I have them planted in planters in front of my house which faces west. I planted them in mid- to late May, deadheaded and fertilized them initially, then a couple of times once I put them in planters. I watered them consistently and deadheaded regularly. In early August, I started to use a fertilizer every two weeks to promote more blooms.
Even a couple of cool nights affects plants in the garden. They get ready to either become dormant, or to produce seeds to ensure survival for the next season. You appear to have done everything right, by providing adequate light, water, and fertilizer. Next year, I suggest that you do not deadhead as much, and see if that makes any difference.
Wave petunias are not supposed to need pruning or pinching back, except to make the plant a little bushier. Perhaps constant deadheading is triggering the blooms to go to seed sooner; this is pure speculation. Sometimes, too, Wave petunias may not enjoy the extreme heat.