Why do some journals allow their papers to be open access for a short period of time after publication?
I don't know about either of the two journals you specifically mention, but in math things like this are sometimes done to attract more high quality submissions to the journal to raise it's profile, particularly for new journals, since academics generally like open-access, and it can be a selling point when choosing a journal.  (From what I remember, with new journals what I sometimes see is that the journal is open-access for its first couple years.)