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I think he's decided the EU negotiating tactics of (a) sticking to what they always said, and (b) not bending over backwards to give us a better deal than we had when we were in, are cementing the leavers' position and turning off wavering remainers. At this point there's a likelihood the leave majority will be higher whereas if there is another ref when the actual deal is on the table, there's a good chance leave will lose.
If there's a second ref now and leave win, we almost certainly won't have a third on the final deal no matter how dreadful it is. If there is a second ref now and leave lose, Farage gets to carry on doing the same lucrative thing hes been doing for years.
Basically win win for him.
I don't think it's a political decision - remember he has no political responsibility (or even interest, I imagine).
Russian investigations getting a bit close for comfort? Realisation that bribes / interview fees will dry out once Brexit / EU off the table.