Asymmetric Routing occurs when the route the packets take to the destination is different to the return route. Causes of Asymmetric Routing are FHRPs (VRRP, HSRP, GLBP) and hot potato routing.
Other causes include static routes like in this demonstration:
Generally Asymmetric Routes aren’t a problem. However in some situations they can be. For example if you have a layout similar to the one shown below you may get Unicast Flooding.
Another issue with Asymmetric Routes is if you’re using a stateful firewall. Replies to traffic leaving the firewall are expected to return though the same firewall. If it doesn’t this will cause it to be blocked.