Have used WD's since the 50's with snap coupler. Never had the hitch come loose. Never heard of this happening till recently when we have more "weekend farmers" using the older equipment. I think if your release chain is too tight or some such it could happen? I have a 1948 WD I plow with now that was converted to the snap coupler in the fifties. I agree with Eldon, I have to back up and release pressure on the "hook" before I can release it. Of course on rough ground the hitch may be alternatly tightening and loosening in the hook. So if your release chain is a bit tight it might release.
Set it up right, should be okay