It'll be because the PIX doesn't recognise that the tunnel has gone down, and therefore still tries the old tunnel and nothing works, until you reboot the PIX or clear down the tunnels. All this does is make the PIX build new tunnels and everything works.
You need to enable ISAKMP keepalives on both ends so that they'll determine that the other end has gone down and reset their own tunnels, allowing new ones to be built.
Use:
crypto isakmp keepalive 30
on the router, and:
isakmp keepalive 30
on the PIX and they'll send keepalives every 30 seconds then and quickly know if the other end has died.
