So what causes the reaction?

The Alka-Seltzer is heavier than the oil and so it begins to sink through the oil. It doesn't react with the oil at all. Once it reaches the water, it begins to dissolve and as it does so, it releases C02 gas bubbles. Those gas bubbles are lighter than both the water and the oil and so they rise to the top. They bring with them a bit of color as they do and that's how you get the "lava lamp"! Once the bubbles reach the top, they pop and whatever water or color was trapped in them sinks back to the bottom and so it continues. The reaction will stop once the Alka-Seltzer has completely dissolved into the water, but never fear, just add another to continue the fun!