lord of the rings - Why did the Old Forest respond to the hobbits?

The relevant part from TT is given in Book 3, Chapter 4 (Treebeard):

'The trees and the Ents,' said Treebeard. 'I do not understand all that goes on myself, so I cannot explain it to you. Some of us are still true Ents, and lively enough in our fashion, but many are growing sleepy, going tree-ish, as you might say. Most of the trees are just trees, of course; but many are half awake. Some are quite wide awake, and a few are, well, ah, well getting Entish. That is going on all the time.

'When that happens to a tree, you find that some have bad hearts. Nothing to do with their wood: I do not mean that. Why, I knew some good old willows down the Entwash, gone long ago, alas! They were quite hollow, indeed they were falling all to pieces, but as quiet and sweet-spoken as a young leaf. And then there are some trees in the valleys under the mountains, sound as a bell, and bad right through. That sort of thing seems to spread. There used to be some very dangerous parts in this country. There are still some very black patches.'

'Like the Old Forest away to the north, do you mean?' asked Merry.

'Aye, aye, something like, but much worse. I do not doubt there is some shadow of the Great Darkness lying there still away north; and bad memories are handed down...

Not everything bad in the world need be controlled by a great evil nor directed towards a single purpose; in this case the trees in the Old Forest are just "bad trees".

There are some other interesting references elsewhere in TT:

'We came down over the last ridge into Nan Curunir, after night had fallen,' Merry continued. 'It was then that I first had the feeling that the Forest itself was moving behind us. I thought I was dreaming an entish dream, but Pippin had noticed it too. We were both frightened; but we did not find out more about it until later.

'It was the Huorns, or so the Ents call them in "short language". Treebeard won't say much about them, but I think they are Ents that have become almost like trees, at least to look at. They stand here and there in the wood or under its eaves, silent, watching endlessly over the trees; but deep in the darkest dales there are hundreds and hundreds of them, I believe.

'There is a great power in them, and they seem able to wrap themselves in shadow: it is difficult to see them moving. But they do. They can move very quickly, if they are angry. You stand still looking at the weather, maybe, or listening to the rustling of the wind, and then suddenly you find that you are in the middle of a wood with great groping trees all around you. They still have voices, and can speak with the Ents, that is why they are called Huorns, Treebeard says, but they have become queer and wild. Dangerous. I should be terrified of meeting them, if there were no true Ents about to look after them.

(Chapter 9)

'So Saruman would not leave?' he said. 'I did not think he would. His heart is as rotten as a black Huorn's.

(Chapter 10)

It's quite obvious that these descriptions match nicely with the observed behaviour in the Old Forest, with Old Man Willow being a particularly nasty example (and, of course, with no true Ents around to look after him, he is quite terrifying and dangerous to meet).