thermodynamics - Cooking pasta: why does adding a lid lead to overflow?

When cooking pasta, some organic foam usually forms on the surface of the boiling water and the situation can be kept under control by adjusting the heat (and/or adding some oil). Covering the pot with a lid, even loosely so, usually leads to the foam overflowing within a short time (removing the lid fixes the problem). Everybody knows that - the question is why?

It seems to me that the only variable that changes noticeably when adding the lid is the vapor pressure but I don't see how that should affect the foaming process.