nitrates constintly rising to very high amounts...

Are they relatively new to the tank?

Then, yes. It's definitely possible. In fact, that's probably the reason. There's a lot of die-off when you transport liverock, whether by mail or even if you drive it home in person. Hopefully you cleaned the rock before you put it in. Because even if you clean it, it will usually still have some dead stuff that will turn into ammonia. Nitrate rising, of course, is due to consistent ammonia production, and it can be removed with water change and maybe some macroalgae.

YEah, some people (I did) cycle their tank by having the tank fully loaded with live rock. Some will die and create ammonia.. and the colony will start expanding. The only risk is that if the rock suffered a lot during the transportation, then there would be high level of dead stuff = ammonia.. which could wipe out all the critters and stuff in the tank. But I found that this process actually takes bit longer than using pure ammonia or shrimp... not sure if that's always the case.

Nitrate isn't as harmful to livestock as ammonia or nitrite but it could result in disasterous algae bloom. So to get rid of it, just do frequent water changes for next few weeks.. Once all the dead stuff on the rock completely decomposes, it should stabilize again.