There are two main causes of fully-double blooms going open-centered:

1. Weird weather in early summer/fall (in which case, once typical summer temps starts, the blooms go back to closed centers. Often open centers happen in the fall, as the weather changes and the plant focuses on tuber-making)

2. Regressed DNA (in which case, the gene that kept centers tight with no visible disk is lost, and all tubers from that plant will have open-centered blooms like its parent. Growers pull plants and toss those tubers, if the parent is consistently open-centered through all weather changes.