The pressure in the well is too much. The engineers are purposely letting the extra pressure vent because they can't suck the oil up to the tankers as fast as it's coming out. It's been like this since they put the cap on. Which is exactly one of the reasons I think this well blew up in the first place. I think they hit a mother load of a well and the pressure in it was just too damn much for any rig to pump from.
Here is a fairly accurate disgram of the well. Note the comments near bottom right of the pic. <img src=http://www.elitetrader.com/vb/attachment.php?s=&postid=2869968>
From the beginning of attempts to gather the oil until the current cap was installed the problem has been: 1. methane mixed with the oil 2. seabed water temp couple of degrees above freezing and the pressure due to water at depth and oil flow pressure, so the methane will coagulate and choke up a collecting pipe in the presence of the chilled water. The problem is NOT too much pressure from the well forcing oil and gas upward. The current cap will not seal against the chilled seabed water. If too much of this chilled water gets inside the cap then ice will form in the cap and oil gathering pipe and stops the oil flow to the surface. Oil and gas pressure inside the cap is kept greater than the water pressure at seabed level. This configuration forces some oil and gas out of the cap at the "seal" at the bottom of the cap and vents and prevents most of the water from entering the cap. What BP is doing to optimise flow to the surface so far, is to close one vent, lower pressure in the riser from above while pumping warm surface sea water down to the riser cap area. You still want to have positive pressure in the cap vs seabed water pressure.