Home brewer here. It's chemistry.

When they put the cap on the beer bottle, they'd already forced a lot of carbon dioxide (the gas that you breathe out) into the liquid in that bottle. They did it by placing the bottle's contents under pressure. And, under pressure, chemistry turned that carbon dioxide gas into a weak and unstable chemical called "carbonic acid" that easily breaks down under stress.

If you were to open the top, and hit the bottle a sharp knock with another bottle, WHAM. That's stress. And the carbonic acid starts to turn back into carbon dioxide REALLY FAST, and you get a foaming up mess.

But it can only do that because it's no longer under pressure. The carbon dioxide needs to go away somewhere so there's room for more to form.

That's why if you leave the cap on, and the carbon dioxide might want to come out, but it really can't... because there's no space where it can go.

As long as the bottle remains sealed, any carbon dioxide that wants to form just increases the pressure in the bottle's little air-bubble headspace, and that increase in pressure just re-dissolves the carbon dioxide back into the water part of the beer.