Step 2 in the reproduction case also includes shutting down the new instance, so I think there is only ever a maximum of two instances at a one time. Seems that the unusual bit is that 70 instances are opened without ever closing the very first instance.
It's not multiple instances running at the same time, step 2 kills an instance. It's one "main" instance running continuously, then opening and closing a second instance over and over.
I can see 70 files or projects open, but not in VS.
EDIT: Ah, seems I skipped the part where the additional instances are closed.