The reason is most likely the frequency with which the data is being sent. When you turn on LSP output, LSP sends the data at the full data rate to the etherdongle multicast address. Your router is most likely trying to forward this out to the interwebs. With other applications (be xlights / vixen, etc.) the frame rate may not be spamming 0 data packets as often. It APPEARS (and I'm not a programmer, but based on packet captures) that LSP is sending the full set of universe data with 0s when you enable the output. With xlights, I know xlights only sends data as needed - as changes occur, plus a packet once or twice per second.
I'd guess you're saturating your upstream bandwidth with multicast in your current configuration.
The recommendation below does address the issue. You also may be able to set your router to block outbound multicast data.