B. walking from Rorikstead over to Swindler's den, another pack, this time plus mammoths, and still wth out my weapon drawn, they suddenly want to induct me into the Skyrim space program. (to be fair, they seemed to have an issue with this, as the bodies of several other recent Space cadets we're laying about 40 yards away)
That sounds like one of the random encounters. Two giants and some mammoths get attacked by a bunch of bandits or poachers (cant remember which.) I’ve run into it a few times along the road between Rorikstead and Broken Tower Redoubt.
First time I was walking along the road there and the encounter literally spawned right on top of me. Still not sure how I survived that, I was at low level that time…all I remember was freaking out because I was just minding my own business when suddenly, mammoths. Mammoths everywhere.
The second time I was approaching the road from over the hill, and witnessed the encounter again. It was in the same spot. I just waited until the giants had killed off the attackers then made my way around them, staying up on the hill and keeping my distance.
They become hostile if they're engaging or have engaged in combat with other enemies eg dragons, bandits or sabercats.
I’ve run into an aggressive giant when I’ve fought a dragon nearby. It was one of those ones that would be normally chill with you, but he kept warning me to back off. Dragon probably ticked it off…
It could vary when mammoths are present. If you take the road south out of Kynesgrove, there’s a giants camp there. They’ll often take their mammoths for walks right across that road. Personally I’ve never gotten too close to the mammoths but the giants are cool with me. I have seen other NPC’s walk really close to them though (sometimes right under them too) and no fuss is made by anyone.