Telling stories by putting episodes on branches of a tree was quite common in those times. Those times? 14th century. If you’ve read Barbara Tuchman’s A Distant Mirror, you know that this was a time of turmoil and calamity. Just think of the great Black Plague in 1348!
That’s about the time when the great Andrea Pisano built the rose window. Times of turmoil often are times of great creativity.