Because you added LR from your existing setup, that LR had a bacterial colony capable of handling some level of Ammonia and Nitrite. It's likely that the existing population grew quickly to handle the new Ammonia (from the new LR) and you will see a rapid cycling process.
It also depends on how bad that new LR was. I have personally added new LR to an existing system and caused no measurable cycle at all because the new rock was in such good shape and the existing LR handled whatever was added. (Adding high quality new LR isn't really much different than adding an additional large fish)
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank: http://www.canreef.com/vbulletin/showthread.php?t=28436
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