Since that is the least destructive. Of course, this depends on hibernate actually working in most cases...
commit 862c40a8e1b5a4051d5ab3b355abe14e05c18427 Author: Richard Hughes <richard> Date: Tue Jul 21 15:46:48 2009 +0100 Change the default to hibernate when critically low on battery power :100644 100644 486ae0a... db202ae... M data/gnome-power-manager.schemas.in