Why might the hyperkalemia stimulate aldosterone secretion?

Because there is more potassium in your body.

Explanation:

Hyperkalemia is a condition that causes more potassium (K+) to be in your body. Aldosterone is a hormone released to regulate potassium excretion This is a key component of homeostasis, wherein your body tries to keep conditions stable and same.

Basically, this means:

hyperkalemia - -> causes more potassium - -> causes release of more aldosterone to remove excess potassium