My oven certainly isn’t airtight, so my best WAG is something to do with convection…
When I crack the door open, a gust of very hot wind comes flying out (enough to make my hair pull back) - it seems far more than merely air mixing.
Can someone please enlighten me? 
warning, WAG ahead, warning
Artificial high-pressure is built up in the oven due to the heat. When you open the door, the high-pressure air immediately rushes towards areas of lower pressure (the rest of the kitchen) and you get the brief “wind”. Same thing happens meterologically, but on a global scale.
awaiting the thump on the head from those who know LOTS more
Don’t underestimate the force of free convection from the 800 degree R air in your oven mixing with the 530 degree R air in your kitchen. Especially when you first open the oven, and the air has a small area to flow through, and thus has a higher velocity.
The interior is not under pressure. Air is released by vents below the surface burners.
I guess my question is, why? Isn’t the “speed of convected air” a constant?
No, or else you would likely not have free convection occuring even.
Free convection happens when you have a fluid which has density gradients. This results in a buoyancy force which causes the free convection to occur. Since the density of the air is directly tied to the temperature, the greater the temperature differential, the greater the density gradient, and thus the greater the buoyancy force.
The temperature differential terms are normally expressed or contained within a dimensionless parameter called the Grashof Number. But I won’t go into that, as we need to start talking partial differential equations to fully understand where it comes from.
Is it a gas or electric oven?