What are the primary causes of network failure to Switch and router?

lots of things may cause failure in network  

some of them are 

the security attacks , wrong configuration , Failed software and firmware upgrade or patches , Link failure

DOS, DDOS, MAC table over flow, ARP attack these are the attacks

Back plane and management plane and data plane traffic should design properly.

Try to limit the broadcast traffic which consume lot of utilization of switch memory using Vlans and private vlans.

Use Cisco access server (out of band) if your switch is down or fail. In is case you can connect the switch and make it up from any where but you need to configure AUX port on your switch with your company modem. check your switches models for aux ports availability. 

It depends upon the type failure, whether device are down operationally or administratively, power outage, port media mismatch, improper cabling, speed/duplex setting, incompatibility between firmware and device hardware, software failure, upgrade or patches are most common reasons.

If it is already implemented and tested it will be stable unless Human configuration change, which can be tracked by Logs and TACACS