I think you need two things. A noble goal to justify invasion in areas of other fractions and reason for unification of other fractions, without making dwarves imperialist.
How about something like this?
Dwarves are a peaceful fraction, although they are always ready to protect their homeland and their people. They lived in peace with their neighbours (amazons and wolves). Their only military problem was occasional raids of bandits. But then, the Cataclysm happened. By use of extremely dark magic their world was merged with another world, with the world of the Empire. The Empire exhausted resources and destroyed ecology in its world, so it used magic to merge its homeworld with another world (by the way, it was NOT the first time when it used said ritual, it explains why its races are so diverse), full of life, resources and potential slaves. The Empire used force to unite various races that hate each other (like orcs and spiders), using "divide and rule" strategy. Said races don't hate dwarves, they are just slaves of the Empire. As the only fraction from their world that has the biggest military potential, the dwarves were forced to defend themselves and the neighbours against the Empire. Against all odds (or maybe because forces of the Empire were overconfident) the dwarves were successufull at defending their homeland. But some of dwarves (including women and children) were caught by the Empire and turned into slaves. In order to free them dwarves gone beyound their borders. They won fight with slavers of the Empire. Then they wanted to make peace with the Empire after that (although trying to make peace with the Empire is just as stupid as trying to make peace with cancer. It's a parasite, it can't live without destruction.), but then their mages noticed something strange: out of sudden grass turned yellow! Many small bugs died. Whole anthills just dropped dead. They concluded that it can mean only one thing: something drinks life energy from their area like crazy. Animals and plants were sacrificing themselves in order to protect the dwarves (the dwarves are very close to nature, almost like elves in other settings) And this "something", according to their research, is located in the heart of the Empire. Instead of conquering them the Empire has decided to drain life energy from the dwarves and their homeland with some kind of dark magic ritual and then get hold on their resources (and possibly enslave survivors, if there will be any). The dwarves must act quickly and reach the heart of the Empire before they become too weak to protect themselves.