Just wondering, because they weren't in fallout 3 IIRC and I never attacked one in the game yet everytime I come across one they are red to me and attack me on sight.

I can verify that behaviour too. It is odd, I agree and perhaps something that modders can address but for now, just RP it that they think you are trying to steal their finds or are spying on their secret places for scavving. After all, you are probably looking more heavily armed than them and they are used to Raiders taking their stuff.

I did come across a small Scavver patrol that I approached and had to defend myself from but one of them did have a note indicating that they are a "shoot first" sort of group.

Based on my experience, they aren't considered hostile until you provoke them and they attack you first. The first two Scavengers I came across I just shot as I assumed they functioned no differently than Raiders. I realized later that my two "murders" the game keeps track of came from killing those two Scavengers. Encountered four of them since then but waited for them to attack me before killing them and it didn't count as murder so yeah.

I remember coming across some scavengers who were not hostile at first and starting spewing lines along the lines of "We were here first!" and then I got too close and they started firing at me. I suppose scavengers are no longer random merchants across the waste as they were in Fallout 3. Just another neutral enemy type until provoked.

The times I encountered scavengers they seem to be territorial, none hostile at a distant but if ya start walking up close they agro, I assume its suppose to be they think you're trying to take there loot.

The wastelands are a paranoid place, if some guy with a huge weapon and armor was running at me, i'd probably try to kill them too.