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.
To ssh://firstname.lastname@example.org/webtools/bmo/bugzilla.git 2e5fc83..6150494 master -> master