Sibling Rivalry
 

I just realized how strange Treebeard's initial dislike of Gimli (and other Dwarves) was. This led me to thinking, "Is there more to this than the axes?"

I searched through my collection of Tolkien's works and found the Ents attacking the Dwarves who held the Nauglimir in Ossiriand, and that seemed to answer my question.

Then I got to thinking again....why did they, who had never before taken part in the war, decide to suddenly attack Dwarves, and not Orcs?

It seems stranger, considering how they are almost siblings. The Dwarves' "father" was Aulë, and the Ents' "mother" was Yavanna.

Thoughts?

The Dwarves like to chop down trees to get wood for their forges. The Ents don't care for this. They were also probably more than a little upset that the Dwarves of Nogrod had just destroyed Doriath, a kingdom of their good friends the Elves. Those were sufficient motivations for them to attack.

As for the Ents not fighting orcs, they did not really have the opportunity. The big forests were away from Morgoth's area and the Ents probably did not ever have a great deal of contact with orcs during the First Age.

If I remember correctly, the Ents were created by Yavanna in defiance of Aule's dwarves. She said that the trees would have need of shepherds, and he answered that the dwarves would have need of wood.

Yes, and the exchange between Yavanna and her hubby was perhaps the only instance of intentional humour in the Sil. But that's besides the point.
Could you say that following the defeat of Sauron the Ents and the Dwarves got on well together?