Why would it need to be a good reason? Especially why would it need to be a good reason, from our perspective?
Hardly an original thought but it's entirely possible that an alien civilization pro-forma sends out an unmanned planet-killer-scale weapon at every alien civilization they detect, simply to avoid the possibility that we might grow up to be hostile to them, or even compete with them for resources.
An unmanned bomb like that wouldn't take many resources and it's possible they would view us as primitively as we view stomping on an ant hill in africa.
Now that's a depressing thought.