How it unfolded
At the pre-race press conference Rossi publicly accused Marquez of interfering with his title fight to help teammate Jorge Lorenzo, and that accusation set the tone for the entire weekend. In the race the two clashed from the start. Rossi repeatedly forced Marquez wide across several corners, and at one slow corner, after a wide line, contact occurred and Marquez crashed out of the race. The win went to Dani Pedrosa, second to Lorenzo, and Rossi finished third. Stewards handed Rossi a three-point penalty, which put him at the very back of the grid for the season finale at Valencia. Rossi appealed and lost. At Valencia he fought back to fourth, but Lorenzo won the race and took the title by five points. Rossi never became world champion again after this.
Why it still matters
This became the event that split the fanbase along national lines over rule enforcement and sportsmanship, and it led to a reassessment of the independence of the FIM stewards' system and its penalty structure. The current three-person FIM MotoGP Stewards panel and the practice of publishing the reasoning behind decisions are results of the tightening that followed this incident.