... let's see what other rational explanation can cover this behavior.
There is already another explanation given above.
European Banks do not only follow the goddess called greed. They also have to stick to some rules.
One of them is called NSFR (Net Stable Funding Ratio). The money those Banks are spending here is not there own, just borrowed.
(Remember: Banks may spend roughly 10 times the amount they have in their safes.)
But due to european laws (Basel II and Basel III soon to come) they have to spend a certain amount in safe harbours, which are rare these days. Thus the prices run crazy. The loss here has to be covered elsewhere. Which is quite easy, as (not sure on the exact figures) they have 5 times the amount spend on state bonds free for other loans.