Click on the keyword field: a list of available keywords will appear, and the first keyword will be automatically selected.
That's not a big issue if you're using the mouse, you click somewhere else and the list disappear.
But you're tabbing through fields with the keyboard, you can't get out of that field: it will add the first keyword, then the second, etc.
I guess the solution would be to not automatically select the first available key.
I cannot reproduce upstream, only on bmo.
*** Bug 1189591 has been marked as a duplicate of this bug. ***
2e5fc83..6150494 master -> master