Why is there a whole lotta shakin' goin' on in Alderaan?

So, it was my understanding that within the Star Wars timeline, we're still many centuries away from this:

So then why, as I adventure around Alderaan, do I periodically hear loud rumbling noises, accompanied by my entire screen shaking.

Is there some kind of ongoing Seismic Event?

It's war. More specifically, cannon fire.

From the codex entry on Alderaan:

Alderaan is a mountainous world of lush forests and grasslands, ruled by an assembly of noble houses and an enlightened and advanced citizenry. A founding member of the Republic, Alderaan has been traditionally admired for its wealth, its art and its thinkers, while the deadly intrigues and politics of the nobility have been downplayed.

Recently, those intrigues erupted into civil war, and Alderaan--now officially withdrawn from the Republic--sees conflict it hasn't known in ages. The Republic is backing House Organa's quest for the crown, whereas the Empire backs the former House Thul exiles. Palaces and fields have become pockmarked by cannon fire, and house armies clash on the battlefield.

Source: http://www.torhead.com/codex/1afjgDN

Another likely candidate for planet-shaking is the planetary defense cannon fought over in the Alderaan Warzone (PvP).