I was playing a 4 player FFA with all normal ai, all of the bots were vasari i was advent, and i went through the tedious task of trying to figure out who to make friends with, i'll call them yellow, green and light green

After jumping back and forwards between green and light green (yellow is a jerk!), and forgetting to reject several of their missions, i was eventually offered a cease fire by light green.

I spent the next several hours building up my relations with light green offering all of my pacts to him, but largely staying in my territory.  I eventually decided that i should help him out a bit so i sent a fleet to help him out, giving him attack planet missions to guide him onto new targets and hapilly letting him conquer the new planets.

We soon wiped out green until all that remained was yellow, and i repositioned my forces as such when he decided to attack the enemy frontline planet with his fleet against a huge number of ships.  a few minutes later and im recieving 'oh god you useless jerk i just lost a capital ship' messages, so i jump my now very large fleets to assist him, the enemy forces fled and i was happy that i managed to capture another planet, a few minutes later and he declares war on me!

Despite seveal attempts to call a cease fire and restart the relationship, and even though i still had a 16/20 relationship with him (and he with me) he refused.  I finished dealing with yellow until i destroyed their homeworld then went over and did the same to light green.

So why did he do it? was it because i wasnt helping him? because i had roughly 4x the forces he had? a combination of both?

It just seems stupid to declare war on a player who is helping you win and has a much bigger force than you, especially when there is 2 player victory on and 1 opposing player is wiped out.