Like many things in the garden, it's a mystery why this plant hasn't behaved the way I've seen it behave in so many other gardens -- most notably the lath house at Far Reaches Farm, where they routinely seem to be over 8 feet tall. Possibly it wanted more compost or perhaps some aged manure? I might try side-dressing my others this coming winter, and see if that has the desired effect on the rest that have yet to flower. Last year it also was covered up for a good part of the summer by flopping foliage from neighboring plants, which have been moved this year into a different area of the garden. Perhaps the lack of sun stunted its growth, or convinced it to flower and reproduce as quickly as possible this year.
I just hope now it produces seeds and offsets like I've heard they do.