Could you tell me why my yucca tree is growing out of the trunk below where the leaves are please .

Question

I have had this yukka for years and it’s never grown shoots below the leaves .

Answer 1

It is probably a healthy sign, of adequate soil moisture and fertility to stimulate new growth. If you have moved it or otherwise allowed more sunlight to reach the stem of the plant, that will also stimulate the latent buds on the stem to develop branching.
If you don't like the look, and you don't want branching to occur, you can prune off the lateral growth back to the primary stem.