This is a reference to the Pied Piper of Hamelin. It is quite witty. You can upvote it.
See, this is all wrong. You need to explain it, this is just pointing out what it's about, that's no help to someone unfamiliar with the tale of the Pied Piper.
The Pied Piper of Hameln is a German folk tale about a guy coming in the town of Hameln (near Hannover I believe) with a magic pipe that made all the rats of the town follow him. He offered his services to the burgmeister for a price, since getting rids of all the rats in a medieval town is a pretty valuable skill. He did that, drowning all the rats in the Weser, but then the burgmeister refused to pay him. The piper was understandably pissed, so he came back a few days later and used the pipe to charm all the children. It's not clear what happened to them, but they disappeared anyway.
So here the joke is that the Wandering Minstrel (which would be the Pied Piper because he's a stranger playing a musical instrument) is making Rats come at night so that his service would be more valuable. This could be qualified as a conspiracy theory, which fits this new meme based on the Dominion card Conspirator.
Fun fact : A popular theory among historians about the Pied Piper is that he's representing recruiting sergeants going around Germany at the time to get people to settle in Eastern lands (notably around modern Poland and Ukraine), thus taking all the young people away from their native villages.