I don't have a name for what you are experiencing, but it is something I see happening all the time.
New pages get indexed, but they don't have a
PR score yet, because that needs to be first calculated which they do during a
PR update.
So in theory, new pages can't rank at all because they have no
PR. So Google seems to rank these pages without having
PR influence the position. The moment these pages do get a
PR score these new pages generally drop because the
PR score they get isn't as high as it should be for the possition they had.
Sometimes I even feel like some of the bigger companies use this and change URLs every now and then so that Google thinks they are new pages again and rank them high.