Maybe you have critters building a nest under there...
No seriously, I have no idea how that package in there could slowly grow.
The airbag is an open design with designated vents to release the pressure almost instantly after deployment, it isn'ta ballon like it is shown on TV. And a deployment will be instantly and not a slow blow over weeks. So there is just nothing there that would make any sense to grow like cancer under your dash.
So, whatever goes on under there it needs to be inspected.
This can be done by yourself if you're a handyman.