[BTS] Barbarian horde moves through without attacking?

[BestEye]

I saw a horde of barbarians move through my territory without attacking or destroying improvements, I'm playing on vanilla and I've never seen anything like this before (and I've played civ 4 for a long time), does anyone know why it happened?

 

[AcaMetis]

Those units were moving to reinforce a barbarian city on the other side of your territory. Their mission was to reinforce, not attack, and so they didn't attack.

 

[danjuno]

Did they spawn through the "Barbarian Uprising" event or naturally? I'm not sure how the units spawned from events work, but with naturally spawned archers, it makes sense that they would act as Metis describes.