Regarding the invasion expansion (when a faction expands into the 8th slot of a system and immediately goes into a conflict with another faction), I think there are a few factors involved:
-The expanding faction musn't have any pending conflict. This means it's not possible to trigger an invasion expansion if you interrupt the expansion with a conflict, and the expansion then goes to the next eligible system.
-There must be an eligible faction in the system, i.e. one with no pending/active conflicts and below X% of influence.
I'll see if I can find some good spots to test this hypothesis.
A faction I have been helping has gone into that kind of state - expansion that leads into a pending conflict on the first day - but when we were expanding into systems with 6 factions to start at times. I don't think that 8th slot is a part of the equation.
So far from my observations it occurs when the new faction that expanded into the system has the opportunity to start a pending conflict given that there is another faction of similar influence in the system and not currently tied into another conflict.
It is kind of chance really, it may be avoidable provided the influence gaps are enough however.