Two things
1.- mid to late inspiratory crackles: when you're breathing in it takes time for the air to flood the lower portions of the respiratory system. so this is why they occur mid to late. The alveoli popping open is one of the later things to happen upon inspiration because it takes sufficient negative pressure to make them open.
2. If I had to guess, the stiffening of respiratory conducting systems results in a greater amount of pressure necessary to open the alveoli, so there is a threshold that is reached where the alveoli POP open and cause crackles/rales.
The second would be my guess but it seems to make sense
