Why do sail boats sometimes tilt to the side?

They have fat sailors I guess

The wind blows the sails from the side, and as the air slides back off the sail it pushes the boat forward. The force of the wind pushes the boat over to the side. You soon get used to everything being at an angle when you are sailing.

It's designed to pitch your beer onto the cockpit floor.

Because the sea does not bank its turns, so the boats have to.

They are "tacking" or changing direction.

from wind whipping?

The keel keeps the boat from sliding to the side. It hikes over as the sail and mast lever it under force. It is thrilling. Everybody works and you will get wet, but you can't beat the excitement. If you need to reduce the hiking up, fall off the wind a little, but maintain your course. Look at it this way; would the roller coaster be so much fun if it only ran a straight, flat track?

Don't knock it until you've tried it.

It’s called heeling, and usually due to the strength of the wind on the sail pushing the boat to one side. While minimal heeling is normal and can aid with speed in races, too much heeling can lead to a capsize on some boats. To flatten out your boat, let out your mainsail slightly until the boat evens out. If you’re in strong winds and this doesn’t work, your crew can “hike up” on the elevated side, meaning they can sit on the rim of the boat and use their weight to even out the boat. Hope this helps :)

You don't think it could perhaps be something to do with the wind ?