What ethical alignment did you choose?
I've noticed the AI tends to be more forgiving when you haven't officially chosen an alignment. They don't treat "Leaning Evil" the same as "Evil," for example.
I've noticed I can get Friendly or Close relations with Evil civilizations if I'm "Leaning Good", but when I officially choose the Good ethical alignment, my relations go down to Neutral. We can be friends again, but it takes more work.
The Thalans are a Neutral civilizations, and those alignments tend to be suspicious of both Good and Evil. Especially if that Neutral race is a superpower. When a race is more powerful than you, if you are not of the same ethical alignment, they can tend to drop relations to Neutral. The strength of your civilization compared to theirs plays heavily into the equation.
As for the Yor situation, I can't say anything with certainty as to why that happened. If you chose Evil, I think (again, not for certain) that Evil civilizations, at first, see each others as competitors, but then can move on to forming closer bonds with each other once the official Evil alignment is chosen. But I don't know.