Theory No. 1
Unable to have his way, Rahul is sulking. His initiatives, including his desire to bring in new faces and axe some general secretaries, are being resisted by senior leaders. He isn't even getting his mother's support.
Theory No. 2
Rahul has had enough of the old guard's resistance to his ideas and the palace intrigues. The publicized sabbatical is aimed at forcing his mother's hand and signaling that he will return to stamp his 'vision' on the party, perhaps as president.
(If senior partymen are to be believed, this theory appears the most likely)
Theory No. 3
Rahul, the 'reluctant politician', is set to bid adieu to politics. He didn't want his departure to be sudden, and is calibrating his withdrawal.
Theory No. 4
After a string of electoral debacles, Rahul is beating a tactical retreat. With many wondering if he has become a political liability, and some within the party beginning to question his leadership, Rahul is moving away from the spotlight and will return once things cool down.