Why did Route configurations change after rebooting Red Hat Enterprise Linux machine?

Issue

• There are two NIC cards in Red Hat Enterprise Linux machine which are connected to two different different networks.
• After rebooting the machine, the route configurations changed which caused outage.
• The machine was not rebooted since long time, and there were old configurations available in configuration file.
• Why did route configurations change after rebooting?
• NetworkManager seems to interfere in the networking.

NetworkManager[1878]: (eth3): DHCPv4 request timed out.
NetworkManager[1878]: (eth3): canceled DHCP transaction, DHCP client pid 733
NetworkManager[1878]: Activation (eth3) Stage 4 of 5 (IP4 Configure Timeout)
NetworkManager[1878]: Activation (eth3) Stage 4 of 5 (IP4 Configure Timeout)
NetworkManager[1878]: Activation (eth3) Stage 5 of 5 (IP Configure Commit)
NetworkManager[1878]: Activation (eth3) Stage 4 of 5 (IP4 Configure Timeout)
NetworkManager[1878]: Activation (eth3) Stage 5 of 5 (IP Configure Commit) started...
NetworkManager[1878]: Activation (eth3) Stage 5 of 5 (IP Configure Commit) failed (no IP configuration found)
NetworkManager[1878]: (eth3): device state change: 7 -> 9 (reason 5)
NetworkManager[1878]: Marking connection 'Auto eth3' invalid.
NetworkManager[1878]: Activation (eth3) failed.
NetworkManager[1878]: Activation (eth3) Stage 5 of 5 (IP Configure Commit)
NetworkManager[1878]: (eth3): device state change: 9 -> 3 (reason 0)
NetworkManager[1878]: (eth3): deactivating device (reason: 0).
NetworkManager[1878]: Policy set 'System eth0' (eth1) as default for IPv4 routing and
NetworkManager[1878]: Policy set 'System eth0' (eth1) as default for IPv4 routing and