What would cause splotchy skin pigmentation changes on my chest, shoulders, and neck that pop up every summer?

Tinea versicolor. This is a benign fungal infection. The fungus prevents even tanning that becomes more apparent upon getting sun exposure, particularly in the summer. You can treat it by applying a dandruff shampoo like Nizoral (ketoconazole) for 10 minutes before showering each day for 1-2 weeks. This will often eliminate the fungus and the skin color will then gradually even out upon additional tanning.