Hat could be the cause of getting subcutaneous emphysema randomly again?

Subcutaneous emphyse: It requires free air generally entering from lungs. Often a lung bleb ruptures putting air into the subcutaneous tissue. If it seals rapidly the source may not be obvious.

Likely a bleb: You can get spontaneous rupture of balloon like areas in your lungs called blebs or bullae. Also, you can get a spontaneous rupture of the mediastinum that could cause it.