My guess, for what it is worth, is that there will be a referendum because:
a) May's deal will fail,
b) Parliament won't through its own inaction allow a 'no deal' exit to happen by default,
c) the DUP won't support a no confidence in the government motion, since currently they've got a kind of direct rule in NI that they wouldn't want to expose to a GE,
d) therefore a GE won't happen over Brexit,
e) there is no opportunity before March to negotiate any other form of Brexit, and the EU won't play anyway,
f) so unless Westminster approve May's deal, which is unlikely, and without a GE, then a referendum is the only alternative to 'no deal', and gives May the only hope of her deal being supported (by the electorate if not MPs).
Therefore, to break the Westminster deadlock, and given the rescind Article 50 option, a referendum involving Remain, May's Deal or No Deal.
Of course, I could be wrong.