#2. He wants to work with younger talent

The Undertaker has been wrestling as a prized "Special attraction" on the cards that he's featured in. We don't blame WWE for doing so as these older generation stars facing each other has generated a good amount of casual interest.
With that being said, The Undertaker probably realized in his match at Extreme Rules that he'll be far more favourable and well-received if he begins working with younger talent. He even teased a match against Drew McIntyre but that never ended up coming to fruition as it was rumoured.
With that being said, The Undertaker probably feels that he has quite a few good matches left in him, which is why he probably signed a new contract. One of the agreements in the contract may even be that he works more with younger superstars than he does with older ones who he has already faced before.