No, placing PS wouldn't have caused this to happen.
The spawner was broken in October... Do you remember this spawner? It had a zombie in it.
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FCucEh.png&hash=f0ae023109be6d75b21cc317ec649f46)
I'm still keeping an open mind about this, so please reply.