What can be the reason, an underwater volcano, tectonic movements?
Geological events
Geological events that alter sea level will be more to do with changing the shape of the ocean bed. For example an earthquake which raises the ocean floor by 2 feet will raise the sea by the same amount. Obviously this would only occur if all the Worlds ocean floors were raised but the raising of one area of sea floor will raise the average sea level, if only by a tiny amount. Over long periods of time this could add up to a significant alteration is sea level. Similarly, a drop in level of the ocean floor would provide a greater volume in which was contained the same amount of sea water to the sea level would fall.
Similar effects would be seen for the growth of under-sea volcanoes, tectonic movements reducing the overall volume of the ocean bed, or landslides from the land into the sea, all of which would increase sea level by displacing the water.
Sea level would fall by tectonic movements which increased the available volume of ocean floor, the destruction of volcanoes by catastrophic explosion such as at Krakatoa or the sea breaking into new low-lying areas of land by erosion of coastal barriers (either natural or man-made).
We can expect a massive geological event?