Another good reason to have a great deal of nitrification bacteria in QT that have been very active recently is that some drugs, such as antibiotic, can reduce the effectiveness of nitrification bacteria.
It is always best to have more than enough nitrificatiion bacterial activity to start, so that even after some compromise there will still be enough.
Ammonia can be released from decay of poop and uneaten food particles, and so can surge.