That's a feature of all Salvia nemerosa/superba/sylvestris hybrids. The more humid the environment, the hotter the nights, and the more they try to grow during night. This causes the plant to get more spindly in the southeastern subtropical states. Older plants do this as the crown spreads. One way to deal with a mature plant is to dig it up, divide it into smaller pieces (with good root systems!), and place the new plants in new locations.

Thanks for the tip -- I often forget about dividing, but I really do need to do it here... maybe the divisions will be happier in a new location, too :)