1. Those following Calvin’s teachings (Protestants) were being persecuted and forced to flee.

2. Those following Calvin’s teachings in England (John Knox, for one) left England to join Calvin in Geneva and others who were like-minded.

Historical background:
Mary, daughter of Henry VIII, was a Catholic. She repealed the laws establishing Protestantism in England and re-established Roman Catholicism. Often referred to as “Bloody Mary” she is noted for her persecution of Protestants. Prominent Protestants such as Thomas Cranmer, Nicholas Ridley, and Hugh Latimer, as well as lesser folk, suffered the heretics’ death: burning at the stake.

In Switzerland, John Calvin was one of the leaders in the Protestant reformation there and Geneva was his home town. Refugees from England made their way to Geneva for refuge. By the middle of the sixteenth century, between one-third and one-half of the city was made up of these foreign Protestants.

This link explains what Calvin taught. He had some differences from the Protestants elsewhere. Among other things, he believed in predestination and also a New Testament structure for the church.
http://www.wsu.edu/~dee/REFORM/CALVIN.HTM