There were two major causes of the Progressive movement.  These were conditions more than events.  

The first condition was the rising power of business elites.  These "robber barons" had gotten a tremendous amount of economic power and, with it, political power as well.  The were seen to be abusing their workers and the public (through things like tainted foods) as well.  This helped cause a situation where Progressives wanted to rein in the rich elites.

The second condition was the presence of many poor people (often immigrants) living in cities.  Middle class Progressives saw the conditions the poor lived in and wanted to ameliorate those conditions.  They also deplored such things as the drinking that working class men did.  This led them to want to reform the poor and immigrants.

The desire to reform the poor and rein in the rich was the basis of the Progressive movement.