Long story short: The wealth of non rich Americans is almost entirely contained in their highly leveraged houses. If housing prices collapse, their wealth collapses. In 2008, housing prices collapsed.
Not exactly. The 25th percentile's wealth was dropping even before the recession, which suggests some additional downward pressure, although I am not sure what that could be off the top of my head.
[1] http://1.bp.blogspot.com/-aGE47oMJAQw/UhZtjjtKorI/AAAAAAAAJz...
[2] http://www.pgpf.org/Chart-Archive/~/~/media/A130E85DDC064B81...
[3] http://www.frbsf.org/wp-content/blogs.dir/1/files/1106bb.gif