Since the addition of Goblins vs. Gnomes expansion the card Undertaker was getting ridiculously often used and played in combination with more minions with Deathrattle condition. And as it is normal in card games it is often not easy to balance the gameplay at the beginning and with each added card it is even more complicated. To think of every possible combo is an enormous task even for Blizzard.
Hence in the upcoming patch Undertaker will get only 1 point added to its attack but not to its health. Which will leave him still a very viable card but could die more easily.
“Undertaker was frustrating to play against,” said Blizzard's community manager Christina “Zeriyah” Sims. “It often gained both attack and health stats significantly above those of other inexpensive minions very early in the game. With this change, we expect Undertaker will still be better than other 1-Mana minions when played in a deck with a Deathrattle theme, but more likely to die in combat against other minions.”
And according to Christina you can expect another changes in the future if needed.