Add the second default route to main route table cause instantaneous reboot without any errors or panic.
After rebooting the route which was added is in main route table.
If to try add first and second default routes during of boot with help of script like

#!/bin/sh
ROUTE='/sbin/route'

$ROUTE flush
$ROUTE add default xxx.xxx.xxx.xxx -weight 40
$ROUTE add default yyy.yyy.yyy.yyy -weight 60

from /usr/local/etc/rc.d ,addition of routes finished successfully.

How-To-Repeat: On 8.0 or 8.1-RELEASE system with kernel options
options         ROUTETABLES=4
options         RADIX_MPATH
add first default route in the main route table (setfib 0):
route add default xxx.xxx.xxx.xxx

after that try to add second default route:
route add default yyy.yyy.yyy.yyy

and you can observe instant reboot.