Why do short pie bases require chilling before baking?
The Good Eats episode on pies suggested this too.  Alton Brown refrigerated the dough before forming, formed it, then re-refrigerated it.

He explained that taking it out of the fridge to form the base would be enough time for some of the butter in the dough to soften and potentially melt.  This would undo all the work put in previously to keep the butter intact within the dough.