if the router runs out of memory or the arp table gets too big.
If you have a static route like 'ip route 0.0.0.0 0.0.0.0 serial 0' the router will have an arp entry for every ip destination.
If you have such a static route it is recommend to change it to sth like 'ip route 0.0.0.0 0.0.0.0 x.x.x.x' where x.x.x.x is the next-hop.
Other than that, I don't see a reason.
Could you provide us more info on the router, ios version and when/how do you detect the problem
