"Crack" can have two different meanings. If you are referring to a crack as in a break, it can happen whenever the conditions are right. It you are referring to an audible "crack" from manipulating a bone at a joint, that is not the bone cracking, It is the movement of air out of the joint, dislodges with the movement. This can happen whenever the space has filled with enough air to be dislodged and make an audible noise when the movement occurs.
