Could be any number of issues, as curtisi pointed out. Anything which interrupts the connection between the agent and manager.
A sure way of knowing is by looking at the insertion time and detection time. If those two times are more than a few seconds apart - there was a disruption in communication and the agent is sending the queued data.
If you are receiving syslog from the routers/switches - are you seeing any interface down/up messages? or route changing messages?
