It sounds as though your petunias are setting seed. As with most annual flowers, once the plant sets and matures a crop of seed, its life cycle is completed and it dies. However, you can keep your petunias blooming profusely all season by trimming off the developing seedpods before they turn brown. Usually this will stimulate new shoots to develop from below the cut. The new stems will bear more flowers. Most modern petunia cultivars require little deadheading. If you’re growing one of the recent introductions and still have trouble with them dying out from seed set, it may be an indication that the plants are being stressed. (Plants often set seed in response to a stressful environment - in an attempt to reproduce before they die.) Container plantings are more likely to be stressed from lack of moisture and excess heat than plants growing in the ground. You might try watering and fertilizing your containers more frequently to minimize stress to the plants.