Lilac flowering lilac bush with one branch producing white flowers how come

Question

Old lilac bush say 20yrs old first year it has produced one branch with
white flowers how and why has this occurred.

Answer 1

This is known as a "sport." This article has some information: https://www.gardeningknowhow.com/garden-how-to/info/plant-sport-mutations.htm
You might be able to propagate the white branches to grow some white flowering bushes!