When an incubator is crowded or anytime for that matter the hatched eggs can cap onto the unhatched eggs a smother the unhatched chicks. This happens more than you would think. As long as you spray those eggs and keep the membranes moist it will not hurt to open the bator. People are just too...
IT does not hurt to take the top off as long as you replace the moisture you loose with a spray bottle with warm water before you put the top back on. If your incubator is full you can take out all the shells and the dry chicks to make room.