Does anyone know why there was a large influx of people to Washington County, Iowa in the 1850's?

My Brown family migrated from Knox Co, Illinois to Washington Co. Iowa in 1855. At that same time, many, many Brown's showed up on the census there that were not present on earlier census reports. Am wondering why they all decided to go to this specific county - was free land available from the government or some other reason? I don't believe my family was part of the Mennonite migration which established itself in the northern part of Washington County, Iowa. Have researched "History of Washington County, Iowa" which doesn't give answers to my question. Anyone have any ideas about this?