It could be a virus that can affect all of the composite family- but i don't recall the name of it. I've seen it happen with coneflowers and gaillardias, too. I kind of like the effect so i've never been too rigorous about culling, and usually it dies out in a yr or 2 and doesn't affect the other plants. Some places will tell you to yank every affected plant right away.

More than likely it is genetic and not caused by a deficiency in your soil.

Here's what Wikipedia says about it:

Fasciation (or cresting) is a condition of plant growth in which the apical meristem, normally concentrated around a single point, producing approximately cylindrical tissue, becomes elongated perpendicularly to the direction of growth, producing flattened, ribbon-like, crested, or elaborately contorted tissue. The phenomenon may occur in the stem, root, fruit, or flower head.

Fasciation can be caused by a mutation in the meristematic cells, bacterial infection, mite or insect attack, or chemical or mechanical damage. Some plants may inherit the trait.

Whenever I drive by a large field of goldenrod or asters I look for contorted fasciated stems - they have a windswept curvy-ness about them that I like to dry and collect instead of normal dried flowers.

I know that with contorted/fasciated pussy willow they propagate it from the mother plant so if you want more of it you can divide the twin flowering plants and keep it going year after year.

Thanks, it seems that the twinning only is on two or three stems on an otherwise large plant.