As water passes through these sulfur deposits it dissolves the mineral. The dissolved sulfur travels in the water until it enters your home. This problem can be constant, or it can come and go with the seasons-or even appear or disappear for years at a time. Because the sulfur is dissolved in the water, its presence may vary with fluctuations in the water table or shifts in the ground rock surrounding your well. Anything which changes what layers of rock your water travels through can alter the amount of sulfur in it.
Drought causes the water table to drop, heavy rains cause it to rise.
Undetected earth tremors cause cracks in the rock, exposing new sulfur deposits or closing up old ones. If your problem occurs seasonally, it's probably tied to the fluctuation of the water table. If you find that you are constantly plagued with the sulfur odor, you may want to take steps to eliminate it. Each of these alternatives has its positive and negative points that will require doing a little homework. Visit the library and look for books and articles on water treatment and well water supplies.