He defended the boss
Just like how the Undertaker and Triple H are connected, so is the Undertaker and Vince McMahon. The two imposing men have both sided together and feuded, with the deadman being buried alive by the boss at the 2003 Survivor Series (albeit with help from Kane).
But despite being the Boss, McMahon is still 'one of the guys' and isn't opposed to some fun. In 2000, McMahon was able to take down Olympic Gold Medalist Kurt Angle backstage. Wanting revenge, Angle wanted to take Vince down and saw his chance, which happened to be near a sleeping Deadman.
The noise of the takedown woke the Undertaker, and seeing his boss being pinned to the mat, he leapt onto Angle, nearly punishing him, before McMahon told him it was all in good fun.