NIC hanged on physical server running RHEL 5.7

Issue

• bond0 suddenly lost the connection (no response for icmp packet), when bond1 and bond2 seems work well.
• After 'ifdown eth0', the primary slave change to eth1 (connection ok) and then 'ifup eth0', the primary slave change back to eth0 (connection ok)
• what is the root cause of faults and why doesn't the fail over from eth0 to eth1 automatically, when this happened?

/etc/sysconfig/network-scripts/ifcfg-bond0

DEVICE=bond0
BOOTPROTO=static
BROADCAST=10.130.3.255
IPADDR=10.130.3.81
NETMASK=255.255.255.0
NETWORK=10.130.3.0
ONBOOT=yes
TYPE=Ethernet
BONDING_OPTS="mode=1 miimon=100 updelay=1000 primary=eth0"

/etc/sysconfig/network-scripts/ifcfg-eth0

# Cisco Systems Inc VIC Ethernet NIC
DEVICE=eth0
BOOTPROTO=none
HWADDR=02:00:00:00:00:1A
ONBOOT=yes
MASTER=bond0
SLAVE=yes
TYPE=Ethernet

/etc/sysconfig/network-scripts/ifcfg-eth1

# Cisco Systems Inc VIC Ethernet NIC
DEVICE=eth1
BOOTPROTO=none
HWADDR=02:00:00:00:00:19
ONBOOT=yes
MASTER=bond0
SLAVE=yes
TYPE=Ethernet