two definitions:

Genotype and Phenotype.

incredibly simplified:

genotype is the exact makeup of the DNA that an organism possesses.

phenotype is what it looks like.

Simple answer is that the genetics have changed, from the bell curve of the phenotype of seedlings produced from broom parent compared to the mother plant.

Let us suppose...

I have a tree of White Pine that has a broom. I collect seed, taken from both the parent plant and the broom.

Most of the seedlings from mom are more or less going to be like mom, with a few outliers creating the bell curve.

Most of the seedlings from the broom are going to be dwarf, or like their parent, as opposed to the original statuesque parent, indicating a shift in genotype corresponding to a change in phenotype.

What Strobiculate said.

I would add that if it is a single broom on the mother plant, the change is most likely a permanent genetic mutation that occured in a bud. The mutation would generally be caused by a random photon passing through the plant, but I would imagine environmental stresses such as parasites/pathogens could also be involved.

In the natural world, mutations give us diversity and are the engine that drive evolution. Unfortunately, they also give us (members of the animal kingdom) cancer. Quite a trade off, eh?