Welcome to the board, Endry.
Very basically, once you reach a certain level on the powergraph (the chart/graph you see as you end a game) relative to the other civs in your game, you will find that just about all of the remaining civilizations will begin to form pacts against your "aggression". This will even be the case if you have been on friendly terms with some of them. The thought is that this is programmed into the game to make the AI a greater challenge to a skillful human player - one whose civ becomes more powerful compared to the rest. Andu Indorin has written a very detailed piece on what factors affect your powergraph rating and I believe you can find it in the "war academy" on this main site.
What this means is that you are probably doing well in your game. However, if you end up attacking a civ that has a pact with another, those other civs will also declare war on you. You will likely also find that these rivals will also begin to share all of their technology as well.
Don't sweat it, however. The AI is really pretty incapable of launching a coordinated attack against you, so I would just keep playing as normal - you're obviously doing something right. Just keep these things in mind as you contemplate attacks and diplomacy.
I am sure that others here will add to/correct some of the things I've written, but that's the beauty of this site: more information than you ever thought possible.
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Diplomacy - the art of
saying "Good Doggie"
until you can find a rock