Key events in Pete Seeger's biography that shaped his activism include his experiences during the Great Depression, the influence of folk music on his upbringing, and his involvement with various social movements. Seeger was a co-founder of the Weavers, which popularized folk music but also faced political persecution during the Red Scare. His commitment to environmental causes and civil rights solidified his status as a social activist. These experiences shaped his belief in the power of music as a tool for social change, leading to the creation of songs that addressed critical social issues.