I have a purple chaste tree and this year it bloomed white flowers
What would cause my purple chaste tree to bloom white flowers this year
It's possible the variety of tree you have is a hybrid. You could have a sport branch that has a different color of flower on it.
A sport is a genetic mutation that suddenly occurs. No one is sure if this is a response to environment, cultivation, stress or simply as common as a human developing a mole. Sports result from a faulty chromosome replication. The resulting defect may occur only once or it may persist in the plant and be passed down to successive generations.
Read more at Gardening Know How: Do Azaleas Change Colors: Explanations For Azalea Color Change https://www.gardeningknowhow.com/ornamental/shrubs/azalea/azalea-color-change.htm