In the end of Avengers (2012) movie,
BLACK WIDOW: I can close it! Can anybody hear me? I can shut the portal down!
CAPTAIN AMERICA: Do it!
IRON MAN: No, wait!
CAPTAIN AMERICA: Stark, these things are still coming!
IRON MAN: I got a nuke coming in, it's gonna blow in less than a minute. And I know just where to put it.
And then Iron Man carried the nuke inside the portal. The nuke hit the Chitauri mothership, which terminated the Chitauri army on Earth.
After they waited for some time for Iron Man to return, Captain America said, "Do it," and Black Widow hurriedly closed the portal.
Why were they in such a hurry? The Chitauri army was down. The situation was fully under control. Iron Man wasn't back yet. Why couldn't they wait for 10 minutes or half an hour until Iron Man got back or for Thor to go look for him and broadcast the Chitauri space location to Heimdall? I mean, if Chitauri started coming down again, they could always stop waiting and close the portal immediately.