I'll speculate that it occurs when packets destined for a destination do not get there. Most people see it via dropped ping packets..

What ping really tells you is that either the packets from A to B are being lost or those from B to A are being lost. From the information that ping presents there is no way to know which direction (and the routing may well be asymmetric) is choking, nor if the problem is at one of the end points. While I really like to use pings (with at least 100 samples) for testing round trip packet loss, there are situations where it will give incorrect results. If the packet loss is data, packet size, or protocol dependent, ping probably won't tell you about the performance to be expected for the application that you're really interested in. On top of that, some systems will filter or throttle ICMP - this really distorts the results. Tony Rall