This is normal. You IP address is stored temporarily in everyone else's (other hosts) host cache. This is why you can reconnect to the network quickly should you be disconnected. In order to establish a connection, they ping you for a while, but eventually give up. Also, if you are uploading a file, the person who was downloading will try to resume the download, once again, using pings. When your client (LW) fails to respond, the other client software gives up and says "Couldn't Download" or whatever. This is nothing to worry about. :-)

To explain why you sometimes have more pings after disconnects than at other times, I will offer you this possiblity. If you were connected to a larger number of hosts than usual, more people will have you in their cache and attempt to make connections to you. And, if someone searched for a file that you have before you disconnected, they may try to download from you. Their search results list is not updated in realtime (due to the attempt to keep bandwidth usage at a practical level), and they will try downloading from you until their client gives up or they just say"forget it." Hope this helps you out! :-)