Seiche is always explained in introductory books as similar to a "bathtub wave". You know, if you get the water sloshing around just right in the bathtub, then stop moving, the water will continue to slosh for a while, and the sloshing will have a big amplitude. My profs have some interesting pics and data on a seiche from a couple of years ago that occurred here. I don't know if they found out what caused it. But the water level in one photos rose several metres very quickly - making it a bit of a worry for people with houses close to the water! They are often caused by earthquakes, I think, though there can be other causes. It's a resonance phenomenon - that is, it depends on the frequency of the force causing it to match the "natural" frequency of oscillation (based on size and depth) in the bay where it occurs. It's a type of standing wave, where the wavelength, or a multiple of the wavelength, exactly matches the length of the bay/pond/lake in question.