It could be a 'sorceror's apprentice' situation: Millenia ago somebody instructed a self replicating, learning robot to retrieve some gifts for his wife's anniversary and then promptly died. Now there's a planet deep in space with mountains of jewelry, dead flowers, chocolate boxes... The robots descend on a new planet, severely interupting commerce as they steal gifts and natural resources for replication. Then they leave to deliver it home.
They could be police bots, designed by a long dead race to prevent intestellar warfare. Ships patrol the galaxies, if they happen to observe hostilities, in comes the robot armada. As long as you don't happen to be caught, you can war all you want of course.
Maybe the robots are just searching for understanding about where they came from, who built them. They have no interest in expansion, but do insist on access to worlds for their archeological digs (even if this means you need to move your cities for them). Something interesting could occur if they succeed.
It could be a single, moon sized robot, travelling from planet to planet to trade for or take by force supplies. It doen't conquer or settle or anything, it just floats through space demanding resources and then moving on.
The robots could be friendly, scientific minded beings voraciously researching all they can. They seem great, if you're willing to help provide resources for their work, they'll share research or tireless automatons. If you look into what they're doing, the big project seems to be a galactic sized particle accelerator. If your scientists look into it...hmm...wait, how much energy is going to occur in that collision? No, you don't really think that could cause a vacuum collapse, do you?