One scenario is coming to my mind and was mentioned by Ari (although in another context): big dump of un-announced code into Qt codebase.

Generally Qt is developed in fairly open way, of course there is "secret" cooking before things are included into public tree but time between disclosure and release of stable version is relatively long so all open-source developers can test new features, develop for them etc. Also those new features are advertised before publishing so there is additional time for preparations. Big dump would break workflow of open-source working. Why? Because Qt packages in most distributions aren't coming straight from Trolltech. This is so called qt-copy maintained by KDE which are TT sources + some KDE specific tweaks. [1]

In case of such big dump KDE developers would have to make tough decision - work overnight and include unreviewed code (risking breaking of core KDE functionality) or shun it. Consequences of both outcomes would be grave. Such situation would really undermine position of Qt by Trolltech in open-source world.

[1] This is because Trolls rarely accept direct patches from outside world (copyrights). Usually they are coming with their own solutions but that can take time while KDE needs solution NOW.