A while back I discussed this sort of thing with some friends and some interesting theories arose. The odd thing that connects Fangorn to the Old Forest, besides the once physical connection, is the attitude of the trees. This raises the question; is this solely caused by age or are there other factors involved? During the discussion, one theory in particular struck me as interesting...
The Entwives were interested in order and, as it were, controlling the plants. I seem to recall Trebeard implying they wanted to, in a sense, bend them to their will. Given the reaction of more sentient lifeforms to similar attempts by Melkor, Sauron and so on, of rebellion and not a little anger, is it possible that the Entwive's 'ordering' of the forest caused at least some of the anger?
I would take this further. If the trees see the Entwives as 'things that go on two legs', so to speak, and the 'ordering' of their lives was seen as some sort of oppression, then other creatures who resemble them will encourage similar reactions. We know the Hobbits were fond of gardens, we know they made the hedge and cut back a lot of trees (albeit, when they attacked). If this theory bears some manner of truth, it seems likely to me that, perhaps, the trees felt that the Hobbits may have been continuing the sort of thing the Entwives were doing.
We know Sam's cousin (allegedly) saw an Ent-like creature prowling the Shire. Even if this is not an Entwife, it may well be the kind of thing that would re-open the trees' old hatreds, if it had been there since the forests 'split'.
Then we have Bombadil; always a spanner in the works. He, again, seems to be one for ordering things, in a way. He can, for whatever reason, control in song the Willow Man, the rain and the Barrow Wight. A controlling force of Bombadil's stature living so close may well be a constant annoyance to the forest (not to mention his songs
), again IF this theory is correct...
All the same, this is all speculation. What say you?