[A_Brandybuck]

Ents are not loving Dwarves, this made Tolkien clear as he wrote Lord of the Rings. But the intention, why Ents and Dwarves have some quarrels, cannot be located in the Silmarillion, because Tolkien invented the actions in the Silmarillion after the creation of the chapters of TTT.
You can read this in one of his Letters from the year 247:
There are or were no Ents in the older stories – because the Ents in fact only presented themselves to my sight, without premeditation or any previous conscious knowledge, when I came to Chapter IV of Book Three

He has created the Ents independently from all of his works from the First Age, etc.
But Tolkien made some connections to the First Age, as he mentioned that Treebeard knows something of Ossiriand.
But since Treebeard shows knowledge of the drowned land of Beleriand (west of the Mountains of Lune) in which the main action of the war against Morgoth took place, they will have to come in.

And then he wrote, that he can foresee, that there was one action, which they made.
I can foresee one action that they took, not without a bearing on The L.R. It was in Ossiriand, a forest country, secret and mysterious before the west feet of the Ered Luin, that Beren and Lúthien dwelt for a while after Beren's return from the Dead

But this formulation indicates (me), that he made this decision afterwards, looking at the date of 1963), so the reason of the quarrel was in Tolkiens conception not there, when he wrote the chapter, but he thought about it later.

Concerning the topic. I remember reading a quote somewhere in HoME, that Tolkien had his very first thoughts about Treebeard as the reason, why Gandalf hasn't come the the departure of Frodo from the Shire. And later then he was replaced by the character of Saruman. But my memory is very vague in the case and I don't find the quote at the moment.

But for the creation of the Ents, how they are, there is a very interesting footnote in his Letter 163:
Take the Ents, for instance. I did not consciously invent them at all. The chapter called 'Treebeard', from Treebeard's first remark on p. 66, was written off more or less as it stands, with an effect on my self (except for labour pains) almost like reading some one else's work.

Then he wrote, that he had some pictures in mind, when he created the Ents.

Their pan in the story is due, I think, to my bitter disappointment and disgust from schooldays with the shabby use made in Shakespeare of the coming of 'Great Birnam wood to high Dunsinane hill': I longed to devise a setting in which the trees might really march to war. And into this has crept a mere piece of experience, the difference of the 'male' and 'female' attitude to wild things, the difference between unpossessive love and gardening.

Tolkien had mind, that the Trees will go to war. From the beginning. That could implicate, that Tolkien wanted them to march against something and this could be only Isengart or Rohan. Due to the circumstances, Rohan is not a very good choice.
Consequently I would say, that Tolkien created the Ents (for the plot) only to march against Saruman, not only Isengart. Tolkien has used and created them as an element against the Istar, ironically against this character, who replaced Treebeard in the conception, if I remember rightly.

Besides the plot, the footnotes tells us more about the background of the creation ("male and female attitude to wild things", etc.)