After Michigan, where Hillary went from having a 20-point lead in the polls to losing the state, many have been concerned about the methodology of polling on the Dem side in the midwestern states, particularly Ohio and Illinois which are voting this Tuesday.
The consensus in the few days following the MI upset was that Ohio was the state Bernie had the best chance of winning of the two. The reasons given were that Illinois was generally more conservative and predictable and less likely to go for the insurrectionist candidate. Also, Bernie's talking points on NAFTA and the like have more impact in a former manufacturing powerhouse like Ohio than in agricultural Illinois.
The factor everyone (except Bernie himself) overlooked: Rahm Emanuel, the mayor of Chicago. Rahm worked for the Bill Clinton administration. He was also a 3-term congressional rep in one of the Chicago districts. Then he was Obama's chief of staff, and a controversial choice. Rahm came to Obama from Hillary; he had worked on Hillary's campaign against Obama. His position in the Obama administration probably helped him get elected in Chicago. He was also a Wall St. banker during the Bush years.
He has come under fire in Chicago in recent months, partly because of his role in the alleged
Laquan McDonald coverup, which involved the Black Lives Matters protestors, but partly just because of his mismanagement of the city.
Apparently it wasn't very well understood in Chicago how close Rahm was to Hillary until Bernie and his surrogates started talking about it more in the last week. Rahm's fiscal sins fit right into Bernie's corruption message, and I think that was all it took for Hillary's "firewall" in Chicago to begin to collapse. I wouldn't be surprised to see Bernie beat the 30% of black support he had in Michigan.
Illinois is one of the most dependable blue states in the country. Take out Chicago with its significant black population, and you've got a red state.
I haven't been studying poll methodology, and 538 hasn't written anything about this yet, but it's possible that this shift in the polls simply reflects a post-Michigan methodology review. But I doubt that tells the whole story. Keep in mind, the Chicago Trump rally got shut down on the evening of the 11th. All this polling was conducted before then.