I did not move over the filter cartridge because I changed filters and the old one would not fit in the new one. Figured there would be enough bacteria on the wood, substrate and water. I guess not. Strange how it just bloomed out of nowhere though after having the water be clear for the first two weeks.
My fish do not seem bothered by this though...
From the sounds of it, it is definitely a bacterial bloom. Not moving over the filter cartridge was likely the culprit. Unfortunately, the amount of bacteria on the wood, substrate and in the water column is negligible compared to the amount that reside in the filter media.
I am not sure why the cycle was triggered so late, but I would keep an eye on your water parameters and do water changes as frequently as possible (it may become necessary to carry them out on a daily basis) to prevent the ammonia and the subsequent nitrites from building up to toxic levels.