Check their intelligence ingame? Perhaps you've managed to bag a Suicidal AI?
Oh, and check their planet count, too.
Foreign Stats-->Stats-->Misc for intelligence, Foreign Stats-->Stats-->Society for planet count.
This is assuming you have a save of the game you can load up, or are still in the process of playing it.
Again, galaxy settings? Tiny? Immense? Abundant planets? Rare planets? These things matter.
And, again, what's your idea of a "rush"? A couple can mean anything from your starting planet + your secondary all the way out to 7 or 8 planets total in 2-4 systems.
Also, how much are you taxing your people?
Economic treaties are worth 10% of the other civ's income, so basically you're saying that they have 20x or more than your entire income.