According to the Wrestling Observer Newsletter, Triple H and Undertaker collectively got together recently and decided it was in the best interest of WWE WrestleMania 27 that they work a program for the show.
HHH was originally scheduled to rekindle his feud with Sheamus, while Undertaker was booked for a match with Wade Barrett. But due to a feeling that ‘Mania lacked that “special” match, the veterans decided to pair off for the show. As there was a desire to involve Shawn Michaels in Undertaker’s match at the show, there was an additional feeling that Triple H, Shawn Michaels’ long-time friend, made the most sense as an opponent.
Sources add that the two lockerroom leaders are putting the bulk of the angle and their match together; unlike many WWE storylines, the creative team will have little involvement in the booking.