Just lucky. There's no way to know it's about to happen but when it takes place there's a sudden loud crack or bang - a bit like a gun firing and echoing - then you see a section of ice at the front just slide down and crash into the water.
If you look at this photo and the next three then you can see the pictures I caught:
https://www.flickr.com/photos/neonbubble/53125764438/in/album-72177720311490419/