This is following the classic scenario for a loss of coolant accident (LOCA). The pressure is now above design within containment. They are keeping it there for a reason, and cannot hold it any longer. Lots of iodine isotope coming out, folks.
In Canada, with the Candu plant, we have a vacuum building for such a purpose. The steam would be sucked into this massive building, and condensed (turned back into water) by a dousing system. The radioactive water would go out into the lake instead of the atmosphere.
Looks like they are waiting for a favourable wind.