Everything you see on the surface is mostly solid. Big old hunks of solid land. Land reaching to the sky (mountains). Huge tubs of solid land holding water (ocean floor). Land is pretty stable.

That's all sitting on burning hot and spinning liquid (magma) at the center of the earth. So we've got solid stuff on non-solid stuff that's moving all the time, and throwing heat under the solid stuff.

All that makes the solid stuff unstable. There are big known cracks between solid masses (fault lines). When the instability of being something solid and hard atop something liquid, hot, and constantpy moving causes enough of a shift that movement has to go somewhere. It tends to present in those cracks.