Suppose a world suddenly is being attacked by frequent tidal waves caused by some event in space. What type of event could have caused this?
Any side effects caused by the event, such as the release off an enormous amount of radiation or a change in the planet's atmospheric composition or color, that would help the plot significantly.
Billions of years ago, when the Earth and Moon were very young, because the Moon was much closer to the Earth, the tidal bore (the swell of water caused by the Moon's attraction) was around a mile high, travelling at hundreds of miles an hour.
Now, if the Moon were to be knocked closer to the Earth (say, by a passing asteroid destabilising its orbit), the tides would be much higher and faster, causing waves that could cause as much (or more) destruction as a tidal wave.
This is possible for any planet with oceans and an orbiting satellite.
Gravitational waves buffeting the planet would provide a plausible source.
Gravitational waves are (in theory) created by rotating objects which are asymmetric. For example, a spinning planetoid with a large dimple (or bump) on its equator. Suppose a wandering planet (a.k.a. rogue planet) entered our solar system. If this planet was such a spinning, asymmetric object that passed close enough to our planet, then we would expect to be buffeted by its gravitational waves. If the planetoid was part of a rotating binary system of two planetoids of different size, then the same effect would result.
Events such as supernovae also create violent gravitational waves. A supernova event need not be nearby to create a big impact. Unfortunately, a supernova would create all sorts of additional effects which would make the gravitational waves the least of our worries.
A rotating binary system consisting of, for example, a black hole and a neutron star could produce detectable results at considerable distance, perhaps many light years.
Gravitational waves have the added bonus of compressing the atmosphere, which would cause the density of particles in the atmosphere to increase, resulting in much more colourful skies. When we look at a sunset, the colours we see in the sky result from looking across the tangent (to the earth's surface) to maximize the depth of atmosphere in view, causing the number of particles seen to increase. It is sunlight reflecting off these particles that create the colour of a sunset. So if the atmosphere was compressed uniformly, then we would see this effect in all directions.
Tides are caused by gravitational fields. Specifically by a gravitational fields that decreases rapidly with distance. Really, any massive object that comes close enough to earth will create a tide, for appropriate values of "massive" and "close enough". For example, a moon-massed object at the moon's distance will create "regular" tides. So for a more catastrophic effect, such an object would need to be more massive than the Moon, and/or orbit more closely to the earth. Ideally, the closer you can get the object to the earth, the better. However, you want to avoid coming within the Roche limit of either object, because at that point, the tides will potentially rip the bodies apart.
Honestly, it's also rather difficult to envision scenarios in which this might happen, without significantly messing up the earth's or the moon's orbits as well.
