We know from Martin and others that stable portals to/from Oblivion are nigh impossible under normal circumstances, so how come?
Another problem is that we know that Pyke entered via the same portal as you did, which would imply that it existed either before the crisis or exactly since the crisis.
How do you explain a part of mania being stable in Nirn?
And as a funny thing I noticed, at least Cindanwe had the same butterfly experience as you did (I think this was the moment you went bonkers btw.) as noted http://www.uesp.net/wiki/Shivering:Cindanwe%27s_Notebook.