I am unclear on how anyone can say that witches broom do not grow not a specific Maple or any Maples. That is categorically wrong. Most Maple cultivars are intentional hybrids because that is effective and/or there are so many distinct cultivars that are available to work with and readily pass on distinct characteristics. It would seem logical to assume that any species that has DNA that is as varied as Acer would be exactly the gene pool that would produce spontaneous chimera even more often than Pine/Spruce/Fir. The differences in SPF are minuscule verses Acer. SPF shows very little color difference. Acer is all over the map. Leaf size same thing. Tree size same thing, too! Twig and bark, same, same.
Witches brooms are spontaneous chimera. A chimera is living tissue that is different than the rest of the plant. It is a change in DNA during cell division wherein the new cell has some change in DNA and it continues to grow thereafter with all subsequent cells replicating the new DNA configuration. These spontaneous changes can be due to radiation, stress, chemicals or something else that affects the cell during division such that the process is interfered with/damaged/interrupted at a critical point and cell division does occur, but the new cell with the new DNA lives instead of dying. It can be a broken link, a link that had been unexpressed, a link that inhibits expression of a characteristic, a novel cross-link, a duplicate link, or any other imaginable kind of change in how DNA operates.
Some species are more mutable than others and thank God for that. Orchids. Hosta. Roses. Rhododendron. Wonderfully variable. And the Queen of trees: Acer palmatum. Keep your eyes open for one near you to layer-off and name after your favorite person!
The witches broom in high trees are shot out of the tree at just the right season to grow cuttings. Easy enough to do with a box of shells in the country, but sometimes persnickety Sheriffs frown on that kind of collecting in the city.