Thanx.  But what if I dont meet certain 'objectives'?  Will the game end abrubtly if I dont colonize enough planets or something?  Caus' thats what happened.

I dont recall ever having an enemy surrender. Perhaps its the map your playing that has that condition allowed. Try making your own maps, and playing against more than 1 enemy. Also, you might try locking teams, as I think when you lock them, the ability to surrender is turned off. (Someone might like to confirm or shoot that down).

Why would you want to abandon a planet that you still have forces in the gavity well ? I cant think of any reason I would do that. As long as the forces remain, the colony should be viable and fully exploited. If the fleet is there as a defence force, then the planet should have fixed defenses there as well. If an enemy wants it, you should be making them pay for taking it, not giving it to them.

What do you mean by 'objectives' ? Sins really doesnt have any, other than being the last man standing.

Nope. You only have to destroy all enemy planets so that the enemy doesn't own any planets anymore.

That's easy to explain. The AI had surrendered to you (only small announcement in the lower left). After this comes a screen asking to end the game or to continue. But if you press your mouse button at the same time, you can inadvertently press the end game button without realizing it, since you'll probably saw that screen for only the fraction of a second.