What caused it?

Firstly, what changed between the 2013 election and today? The LSE's European Institute notes a major shift in the top political concerns of the Italian public between 2013 and now. In 2013's Eurobarometer, economic issues dominated, and migration (4%) and terrorism (1%) were non-issues. In late 2017, migration (33%) and terrorism (22%) were now rated among the top three political issues. While economic issues were still present, less voters named them as top issues. The number of Italians citing the economic situation as a top issue declined from 42% to 22%.

The European migrant crisis beginning in 2015 would appear to be the cause of this. Over 600,000 migrants from North Africa and the Middle East have arrived then in Italy.

Pew has found that 69% of Italians have an unfavorable few of Muslims in their country. Italians from across the political spectrum have generally unfavorable views of Muslims, with 50% of left-wingers and 65% of centrists reporting unfavorable views. Only 18% of Italians think that growing diversity in Italy is making it a better country.

Given the views of Italy on these matters, the migration situation has been politically explosive. Parties with populist positions on immigration, MS5 and Lega, saw large increases in their vote share.