In Stranger Things 2, they say the portal takes a lot of energy to close, and that only El has the power to close it. But in the first season, portals to the Upside-Down crop up all over the place- in the Byers' home, in the tree in the woods, and by the swimming pool. What's the difference between the Hawkins' lab portal and the other ones?
The wiki states that the smaller portals were inter-dimensional "wounds" that would close/collapse quickly after forming, and that only the Demogorgon could make them because they took a lot of energy to create. Presumably the "demo dogs" are too weak to make portals like this, which is why they dig the tunnels instead.