An accident is something that happens when you are not expecting it, and that you didn't mean to have happen. The opposite of something happening by accident is happening deliberately, or on purpose.
An earthquake is a sudden release of energy under the Earth's surface, like a sudden cracking along a fault line. Earthquakes can sometimes be caused by volcanoes. Earthquakes can cause severe shaking of the ground, which makes building buildings fall down and roads and bridges to crack and break.
When something explodes it suddenly gets bigger. Explosions usually make a loud noise and make a lot of heat. Popcorn popping is an example of a small explosion, and a bomb going off is an example of a large one. Large explosions can create so much force they can reduce buildings to rubble.
A stampede is when a large group of animals suddenly all run in the same direction at the same time. Stampedes usually happen when something scares a herd of animals.