"Galls are fascinating and intriguing structures. They are parasites on plants. Each gall is an abnormal growth produced by a plant host under the influence of a gall causer. The structure produced by the host isolates the gall causer but the causer is still able to draw nutrients from the plant as its food source. Several organisms cause galls including gall wasps, gall midges, gall mites, and a few examples of aphids, moths, beetles, micro fungi, bacteria and nematodes. The causer lives within the gall for at least part of its life cycle. The study of plant galls is called Cecidology.
I think the strange structure you discovered with your team on Sunday morning is probably an example of a gall caused by the bacteria Agrobacterium tumefaciens. Initially, from Adrian’s description I thought it may be a fungal gall, Taphrina betulina which is common on birch and known as the ‘witches’ broom’. I wonder if you might have seen this somewhere, it can be quite common. Adrian assured me the tree was not birch"
So, looks like it was probably caused by a bacteria, thanks Tom!