In the case of shootings, one must talk about what may have caused shootings to start noticeably increasing in around the 1970s, to the point where a single year started having as many shootings as all the shootings prior to the 20th century combined.

Fun fact; the first documented school shooting was perpetrated by some Lenape indigenous people, in Pennsylvania in 1764, during which attack they also beat to death all of the students and scalped everyone.