There was a super deep channel in the middle of the slide path that looked like it had been formed by a massive ice chunk that fell from the cliffs above. We believe that this is what caused the avalanche because above this apron the snow was very stable and showed no signs of propagation. Also, we tested out the stability of the slope just to the skiers left of the slide path and it also showed no signs of instability so we skied it. When we got to the bottom and more closely examied the debris, we could see the massive ice chunk that I think likely caused the whole thing. It can be seen in the pictures taken from below.