Dropped Destination Unreachable log messages are caused when a router or other device on the Internet blocks a connection for some reason. When a router will not allow a specific connection through based on some specific criteria, it will often send a separate message back to the originator to let it know this is happening. The SonicWall drops this new incoming connection and logs the event. It is common to see these messages occasionally. They are not a sign of any kind of attack. If you are seeing a large number of these messages, there may be a configuration problem or a protocol blocked upstream from your network (not with the SonicWall itself). You need to contact your ISP to determine why this is happening.
As an example: If a function such as NTP is not working properly, look for periodic destination unreachable packet dropped messages and the source address of the packet. The source address might be from an upstream ISP router blocking the protocol.