Why is Auntie's Tree Blowing Bubbles?

I looked out the window and saw a white patch at the base of Auntie's Tree and had to go out to see what it was.  It was raining quite steadily and at first it looked like some insect eggs but no, it was literally flowing down the trunk like a wee waterfall.  Of course, I had to rush back in and consult Dr. Google. It is, believe it or not, a kind of soap bubbles.  In the 22 years the tree has been here I have never seen such a thing.  It was gone once the rain stopped.  You can read about stem flow mixing here...