Witch's Broom is a German word - derived from 'Hexenbesen' - Hex for witch and besen for broom. The witch's brooms in the trees are a dense mass of small shoots which can be caused by many things - fungi, oomycetes, insects, mistletoes, mites, nematodes, phytoplasmas and viruses. Infection often ocuring as a result of damage to the branches.
We also came across a Snake's Head Fritillary (see bottom picture). These can be common in people's gardens as planted exotics - but in the wild they are a rare sight. Found this one growing in a walnut orchard.