We can’t know what reasons they would conceivably have, but based off of our own history we can speculate. We explored new locations for resources, scientific advancements, the sake of exploration, conquest, etc. Is it so hard to imagine they are part of a colony ship and decided to “restart” the ecosystem than deal with hostile lifeforms?

You are likely correct with the last point though. Do you think there is a galactic reality TV show depicting the Earthling shenanigans?