Why do Beyond Earth alien units attack settlers/explorers?

I have just purchased Sid Meiers Beyond Earth and am loving it even with its somewhat negative reviews. However, there are some differences that I can't understand. From what I know, aliens are initially indifferent to humanity, or basically neutral. That being said, I have had several instances where I was moving an explorer across the landscape when they were suddenly attacked by several alien units at once and destroyed in one turn. The attacks usually involved at least one Manticore and a pack of Wolf Beetles. In most of the attacks, I was nowhere near an alien nest, and I have never killed or even attacked an alien unit/nest (and have no plans to as I'm pursuing Harmony full force).

In addition, the same thing has happened with one of my settler units. I was moving it through heavy alien territory with a Marine escort (again, no nests within several tiles) and decided to embark to avoid aliens. I embarked the Marine first, and then tried to embark the Settler onto the Marine, stupidly assuming that since they could stack on land they could on water...but no. The Settler remained on land and was promptly destroyed by a Wolf Beetle.

I have been trying to figure out why this has been happening. Yes, I may have come too close to a nest a some point, but that shouldn't remain in effect for a long time, will it? Also, the aliens are still neutral towards me (ie. green). I'm assuming that the aliens can't detect if a unit is a Settler and feel threatened, but it might be possible. If anyone could shed some light on this that would be great!

In general, any information/clarification about aliens and their relations would be great! I have read the Civilopedia, but still cannot find the answer. Thanks!

EDIT:

I can see Firaxis idea that an alien planet would be infested with other life forms that may or may not appreciate being disturbed, but I would have liked a bit more warning. From now on, I will stack at least a Marine with everything, as they always appear to be unmolested this way.