Lars Knoll explained on the Digia Qt Blog their reasoning for adding the LGPLv3 license option for new code:
Trolltech was involved in talks with the Free Software Foundation (FSF) when the GPL v3 was created, and we added this license as an optional license for Qt after it was published by the FSF. Finally, in 2009 Nokia added LGPL v2.1 as a licensing option to Qt.
The spirit of all GNU licenses is about a strong copyleft, giving users rather strong access and rights to the source code of application and libraries. It was always meant to protect the users’ freedom to modify the application and underlying libraries and run the modified application.
In many people’s opinion there is, however, a loophole in the LGPL 2.1, where it doesn’t clearly talk about running the applications using a modified version of the library. Even though it violates the spirit and intentions of the LGPL, this loophole has been extensively used by companies that create locked-down devices. If devices use LGPL v2.1 software, the user may not be able to install modified versions of the library on the device and use it together with the other software that is installed on it.
We also consider locked-down consumer devices using the LGPL’ed version of Qt to be harmful for the Qt ecosystem. The device is not open to third party developers and thus doesn’t contribute in extending the size of the Qt ecosystem and the range of devices that can be targeted by software developers using Qt. In addition to not contributing to the ecosystem, it doesn’t fund the further development of Qt.
For these reasons we believe that LGPL v2.1 is not protecting the users’ freedom as it was intended by the Free Software Foundation. To account for this, the FSF created version 3 of the LGPL, a license we feel is legally formalizing the intentions of the earlier version.
Starting with Qt 5.4, all existing modules will be LGPLv2.1, LGPLv3, and the commercial license. Some new modules though will be just LGPLv3 or the commercial license, such as the new QtWebEngine module.
Qt stakeholders wishing to learn more about the LGPLv3 option can read the aforelinked official announcement and there's also a KDE blog post.