When was this shrub planted? Kerria tends to be a suckering plant, known as a 'caning' shrub, meaning that it produces new stem growth ('canes') from the root crown. Given more than a couple seasons in the ground, one typically has a pretty decent stand of the stuff. Often, more than one wants :-))

Kerria tends to be tall and spindly - that's kind of its growth habit - and the more shade it is in, the lankier it becomes. And the cultivar 'Pleniflora' is known to be lankier than the species. Cutting back to the ground periodically can be beneficial. It also does better in a richer, well-draining soil.