I'm surprised I didn't find it mentioned anywhere in this thread of many years, but surely others have encountered the weird behavior of the Goth spearmen accompanying the Goth settlers, who will stupidly go to war with you, hopelessly attacking a unit or even a city that might be in the way of where they want to go (or wanted to settle, but you got there first). Or they will take one of your workers on a mountain space where they happen to want to go, even though the spearman cannot enter the mountain space unlike the settler.
I think having a settler which is like a unit, which has to be killed and gives no workers when captured, which can enter terrain its accompanying unit cannot, is making for some very weird bugs in behavior. I have been playing a very frustrating game where the Goths are my next door neighbors, and though we are on excellent terms, if one of my units happens to be somewhere a settler might want to go, they go to war with me. I think this is a bug that should have been noticed and remedied long ago, because it is plainly irrational. It is vexing to try to predict and tiptoe around this weirdness in order to keep an AI from randomly going to war with me for reasons not based at all on any other normal factor.