Like most materials, the material glass expands when it gets warmer. When you place your thick glass in hot water, the outside of the glass gets hot right away. Glass is not good at transferring heat, so the inside of the glass gets hot later. Due to the thickness of the glass it takes some time until the warmth has reached the inner layer of the thick glass. This uneven distribution of warmth causes the outer layer of your glass to expand first. Because the inner layer is not yet warm and does not expand yet, a strong tension is being grated within the glass layer. And if you're unlucky, your glass cracks because of that!