You could run into this problem with any type of server really. Even an ordinary web or FTP server serving something popular might have who knows how many people trying to make contact - they're not always aware that the server may be behind a firewall or no longer available for whatever reason (imagine, for example, what would happen a popular domain like microsoft.com suddenly mis-configured to resolve to your firewall's address).

The only difference with a P2P app like gnutella is the peer can make itself known very quickly and create a large sudden demand. Still it depends on which gnutella peer they're using. Some have better firewall support than others, and how well the user understands what they're doing makes a difference too.

The gnutella protocol can work using outgoing connections only and be firewall friendly, but if someone sharing very popular files mis-configured their peer to report your broadcast address as their external firewall address you may see this kind of traffic. Normally this kind of setting would be intended for use with port forwarding on the firewall itself, but perhaps your user didn't understand what it was for.