For the rev xml packages, the default behavior is to only collapse if it's the currently selected category.
You can just go to the skins xmlpackages folder:
{web folder}skins/skin_{id}/xmlpackages
and modify rev.categories.xml.config
scroll down to the bottom and change
C#/VB.NET Code:
<xsl:if test="count(child::Entity)>0 and (EntityID = $CategoryID or descendant::Entity/EntityID = $CategoryID)">
to
C#/VB.NET Code:
<xsl:if test="count(child::Entity)>0 ">
basically just removes the parent checking, you may also need to do this to the manufacturer and section xml package if you use them, just different parameter name.