bad fold?

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GF on river, tough call on turn.

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what can he possibly have?

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Well once everyone folded to him on the flop he may have called with J8, 84, 4x, etc. I think 4x is most likely because of his apparent value bets on the turn and river.
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was he on a draw? 9 T, 5 6? so bluffing on the river

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No, not most of the time anyways. Unlikely with the effective stack sizes he is trying to push you off a hand from the SB, he is betting for value on the turn and river IMO.