For any lenders who priced late enough in the morning or who repriced near the earlier MBS highs, negative reprice risk is starting to pick up again.  Fannie 3.5s are now down 5 ticks from the last stably-held plateau of 102-26 (currently down to 102-21).  While this doesn't guarantee negative reprices for this crowd, it does mean they can't be ruled out completely. 

There is no overt cause for the weakness apart from the fact that it picked up as the 3pm CME Treasury close approached (not an uncommon time of day to see bond market volatility).  If that's not satisfying to you, here's a random Greece-related headline to blame things on:

RTRS- GREEK PRIME MINISTER TO MEET JUNCKER, ECB'S DRAGHI, IMF'S LAGARDE ON WEDNESDAY - PM'S OFFICE