Unless you are actually adding these fields to a table definition (as opposed to a TO realtionship), I would not expect this, and it's not happening on my files when editing relationships with multiple- or single-key fields.
I am also unaware of any standard keyboard shortcut which would do this.
Can you test a copy of your file on a different computer, or post a copy of the file for someone to test for you on another FM12 installation?
Also provide some info on your OS version. I am working with FMP12v2 on a Mac OS 10.7 machine for my testing, so my OS-level shortcuts may be different, though I am unaware of any FM shortcut for either platform which does what you describe.
I'm running mac OS 10.7.4 and filemaker 12.v2. So far, I can only get it to happen with one file, so I ran a recovery on it to see if that's the problem. The recovery found no problems, but the behavior is still happening consistantly with this file. Here are some other details.
at the time it's happening, there are 2 windows open for the same file. one is a floating document window, the other is a normal document window. The normal window is the one that keeps adding all the fields. Other possibly significant details: This is a conversion project and the table/layout that is misbehaving currently has 1478 fields (I am in the process of moving most of them into other tables). It will happen after just opening an edit relationship dialog box, even if I don't change any of the fields in the relationship. If there is only one window open, it doesn't happen.
when i try to get it to repeat the problem on a newer, non-converted, small file, it doesn't happen. so i'm guessing it either has something to do with being a converted file or having so many fields in the table in combination with new fm 12 advanced window styles. After I get the field count down to a reasonable amount, I'll report back if the behavior continues. in the mean time, i'm turning off the preference because it's a serious pain to remove 1400 or so fields from the layout every time I modify something in the relationships graph.
And meanwhile, until this gets fixed, you can turn off the Add New Fields to Layout option in the FileMaker Preferences under the Layout tab.
That's supposed to stop the adding of new fields to the open layout during schema changes.