If you open 1 part and via menu File > find references what do you see?
I checked that: only the references that should be there; none the 50 or so spurious parts!
I have found a fix (so far) in that I checked everything into PDM except for the new parts I was trying to open and then deleted everything out of my working folder except for those new parts. They now open without trying to open anything else but I don't know what caused the problem in the first place or indeed, if it will happen again when those spurious parts ever get into my working folder again (i.e. when I open the assembly that they are part of).
So, basically by deleting them from your active folder, they don't get opened again.
Hard to tell what caused it.
If you get that kind of behaviour again in the future I would zip those files and make an rx of it. The SolidWorks guys can look into it much deeper.