When Eric Reid of the Carolina Panthers stepped over the sideline and onto the playing field to instigate a heated pregame confrontation with Malcolm Jenkins of the Philadelphia Eagles, the Panthers safety figuratively crossed an important line regarding civility and tolerance.
Based on multiple reports including The New York Times and ESPN (please read these articles if you are not familiar with the details of the backstory), the root cause of the conflict is the two men disagree on the best approach to address matters of social injustice and inequality.