I've heard scientists explain that large earthquakes can cause changes in the Earth's rotational speed (on the order of milliseconds) and tilt. I wonder if they might have that backwards. Could slight changes in the Earth's rotation and tilt trigger earthquakes? I assume they check the data before and after a quake, but to what degree of accuracy? Do they know for sure which occurred first? The quake or the change in planetary movement?

It seems intuitive that what happens on the crust is mostly caused by what happens under it. So what changes the motion of the material under it? Radiation? Gravitational forces? A combination of factors?

I'm not sure if a pursuit along that track would lead to anything useful, even if correct. But perhaps folks much smarter than me could investigate the data from larger quakes and compare it to conditions throughout the solar system to see if there are unlikely coincidences. Maybe they could find an object or two we are currently unaware of, or some connection between earthquakes and something we currently see as completely unrelated. I'm inclined to believe that nothing in the universe is completely unrelated to anything.

Not really my area but I think you're edging towards convective currents driven by heat as the answer. The heat energy comes from radioactivity plus leftover heat from the creation of the Earth. However, Arp has touched on this topic, speculating (I think) that matter creation may be a factor. The fact that continents move around led to two possibilities being proposed: plate techtonics and expansion of the Earth. The second of these two ideas has been discarded by most scientists. External causal factors are harder to establish, perhaps because in any given week there are a large number of earthquakes (only the major ones affecting populated areas are reported in the news). It was interesting to learn via the coverage of the recent events in Japan that earthquakes redistribute stress in the crust, lowering the risk of a quake over here, increasing the risk over there.