[Wanderer]

Probable causes of AI anger:

1) you own/have claimed a bunch of systems the AI considers to be 'rightfully' his. Check out the Borders tab in the Empires window and look for clashes.

2) Mega Evil Empire (MEE) effect. Once you get too large compared to the other races (it's worked out as how large the score of the race in first place is compared to the score of the second placed race) they'll all gang up on you.

From settings.txt:
AI Uses Mega Evil Empire := True
AI Mega Evil Empire Threshold Score Thousands := 500
AI Human Mega Evil Empire Score Percent := 170
AI Computer Mega Evil Empire Score Percent := 250

To translate: if you have more than 500,000 points (score not race experience), are in first place on the score table and have 70% more points than the second place player, you'll be public enemy number one.

3) Some alien races just don't like you!

Sounds like you've hit MEE if everyone has suddenly declared war on you. This is a deliberate part of the game to prevent a human player from running away with the game. If you're quite small when this happens it must mean the AI races are not doing very well.