Mt. St. Helens erupted catastrophically in May, 1980--products from that eruption are visible on the canyon wall in this photo.
So, this canyon, which is about 500 feet deep, has formed during the last 20 years!

Why is the erosion so fast?  --> Have a look at the material that makes up the canyon, visible in the foreground.  It is mostly loose, unconsolidated pumice!

For more information on Mt. St. Helens, visit the USGS Cascade Volcano Observatory.