As the planet was already inhabited, by several anarchist and outcast groups, they sent messages to the ships sending off the probes, mostly on the lines of 'There are people living here, we just want a place to ourselves, just leave us be'. Meanwhile, several of the Necromantic groups and cults that lived upon the planet went active, raising new undeathly creations or commanding their already-existing ones to move into defensive positions around the planet, in order to protect people or divert fire to unoccupied areas as diversions, assuming they were attacked by these new ships for no reason.
It has, after all, happened before.
In addition to the various Necromancers, other mages of...Less-than-morally-white magic also added their own defensive spells around the planet, but the Necromancers provided the bulk of the defenses, even so.