Law 20 of the International Rugby Board states the laws regarding the scrum, and 20.1 specifies the forming of the scrum. Amendment 21 to this section states the following:
CURRENT LAW
20.1 Forming a scrum
(g)
The referee will call “crouch” then “touch”. The front rows crouch
and, using their outside arm, each prop touches the point of the
opposing props outside shoulder. The props then withdraw their arms.
The referee will then call “pause”. Following a pause the referee will
then call “engage”. The front rows may then engage. The “engage” call
is not a command but an indication that the front rows may come
together when ready.
AMENDED LAW
20.1 Forming a scrum
(g)
The referee will call “crouch” then “touch”. The front rows crouch
and, using their outside arm, each prop touches the point of the
opposing prop’s outside shoulder. The props then withdraw their arms.
Following a pause the referee will then call “set” when the front rows
are ready. The front rows may then engage. The “set” call is not a
command but an indication that the front rows may come together when
ready.
While the official page for the amendment does not have any information as to why this change was made, the wikipedia article on scrum claims that the change was to speed up the scrum:
"Pause" has been removed in order to speed up the scrum and to
minimize resets due to collapsed scrums.
I found another article on "The Australian" which claims that the change in directions during scrum is one of the changes on trial basis for 2013, and that
the motivation was to decrease the risk of injuries as well as speeding up the game. The article features opinions from several professionals as well, good read.