In this case, we are talking about major league pitchers deliberately throwing at batters, called "Hit By Pitcher" or HBP. In 1994, the HBP rate exploded. Why?
Before 1994, if a pitcher deliberately hit a batter, the batter's team would retaliate.
After 1994, the rules changed to a "warning-then-eject" rule. Once the “warning-then-eject” rule was implemented, teams effectively got a free HBP, with no fear of retaliation. Team A hits team B, then both teams get warned and team B can’t retaliate without automatically losing their pitcher and manager! They de-incentivized retaliations to such a degree that they actually incentivized the first HBP.
Article summarizing the history of the HBP rule and with nice graphs of the HBP data:
HT: Eric E.