Both never saw eye to eye after Morrison was promoted and now their feud reached breaking point.
Morrison advocated for holding Overwatch together while Reyes disagreed.
Although we don't know exactly what his agenda was, he is likely linked to attempts by rogue elements in Blackwatch to bring down the organisation.
At Overwatch's Swiss HQ the two men fought each other resulting in an explosion that destroyed the base and reportedly killed them both, although their bodies were never found in the wreckage.
That act was the death knell for Overwatch and the organisation was disbanded 20 years after the end of the Omnic crisis. Its members took different paths.
Many like McCree became mercenaries, a group including Mercy and Reinhardt carried on their heroism without the organisation while others like Winston watched and waited for the days of heroes to return.