Firstly, Corsairs are more likely to choose harvesters over Zoners being that they aren't exactly how you say "Neutral" with them these days. All sorts of things have brought the Zoners down to unfriendly terms with some corsair factions. I also believe (correct me if i'm wrong) that it may have been some time before or recently a corsair faction was hostile with zoners.
Secondly about harvesters. You may want to look at the equasion from both sides. Basically the Harvesters want certain piece of information from the zoners called the "alpha code" which the zoners didn't want to give up likely. So the Harvesters launched an attack on Gran Canaria and started harvesting zoner ships to obtain traces of info on it. Then basically the zoners called a ceasfire for a deal to be done on the alpha code in which they set a trap.
So technically the zoners arent to blame for the Harvester - Zoner relationship, entirely.