It's because when you're under water ALL air is pressurized by the crush of the water around you.
When the scuba tank releases air into your lungs (and therefore your sinuses, which are connected to your inner ears), it's still partly pressurized, meaning it's at a higher density than it would be up at the surface. As long as you stay down, your inner ear matches the pressure level of the air at that depth. When you start to surface, the air in your lungs expands gradually. Same for the air in your inner ear, but your ears naturally bleed the over pressure back into your sinuses (unless you have a cold or something, in which case resurfacing can be really painful and must be done more slowly). It's just when you're going deeper that you need to force more air into the inner ear because what's already there gets compressed and your sinuses function like a check valve preventing more from getting in without intervention.
When the scuba tank releases air into your lungs (and therefore your sinuses, which are connected to your inner ears), it's still partly pressurized, meaning it's at a higher density than it would be up at the surface. As long as you stay down, your inner ear matches the pressure level of the air at that depth. When you start to surface, the air in your lungs expands gradually. Same for the air in your inner ear, but your ears naturally bleed the over pressure back into your sinuses (unless you have a cold or something, in which case resurfacing can be really painful and must be done more slowly). It's just when you're going deeper that you need to force more air into the inner ear because what's already there gets compressed and your sinuses function like a check valve preventing more from getting in without intervention.