An avalanche is when there's too much snow on a mountain and it may fall down and cause quite a big trouble. It's one of the natural disasters which is cause by the nature. Avalanche can actually kill many people and often mixed with ice and debris. Most avalanche is formed by weak layers of snow and then grow into a giant ball.
The weather and tempeture changes everyday and so it depends on it if it is going to cause an avalanche. It also depends on the slop angle too, even earthquake can cause avalanche.