#1 The Undertaker has been known as the locker room leader (kayfabe reason)
From a storyline perspective, I think that this is the most obvious reason among the ones that I have outlined. The Undertaker may be one of the most celebrated WWE Superstars of all time, but as any fan knows he occupies an even more special place in the WWE roster.
As has been mentioned time and time again in the WWE annals, The Undertaker was the locker room leader. Which means that it was his job to keep the locker room in check, and when they got out of line, he would take necessary action.
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Now that Shane McMahon was misusing his authority, it made all the sense in the world for The Undertaker to set things right. Hence, he interfered and challenged the tyranny of Shane McMahon.
Why do you think The Undertaker returned, dear reader?