Why does my oleander’s leaves go yellow and drop off?

Question

It has flowered really well. This is my second summer to have it.

Answer 1

This could be normal pest invasion that can be treated with horticultural oil. https://www.gardeningknowhow.com/ornamental/shrubs/oleander/oleander-dropping-leaves.htm

If you see the condition overtaking the entire plant, you could have a problem with leaf scorch, which doesn't have a cure:
https://www.gardeningknowhow.com/ornamental/shrubs/oleander/yellowing-oleander-bushes.htm.