Nitrospira (the nitrite-eating bacteria) can go into a dormant stage when conditions are unfavourable. That's why it's possible to bottle those, but not the ammonia eating bacteria. It's quite likely that some died and some "went to sleep" and have now woken up to start work again. Dilution of nitrite would be virtually instantaneous so is not an explanation in itself, though it will be a partial factor.
The stones and resin could not transfer nitrite. If they were from another tank and had organic residue that could cause ammonia and then nitrite, but you had no ammonia spike.
EDIT: Just re-read and find I had the names of the bacteria switched - nitrospira are the nitrite eaters, nitrosomonas are the ammonia munchers.