Because of the issues between KDE Plasma’s KWin window manager and the next-generation Wayland display server that the KDE Neon devs encounter with the Qt 5.8 release of the widely-used application framework, they decided to jump straight to version 5.9, which they say it’s more promising. You can consider this announcement a call for testing, as the KDE Neon development team needs your help (if you are a Qt software developer) to try out the latest Qt 5.9 packages on the KDE Neon Developer Unstable Edition, and report any bugs, issue, or annoyances you might encounter, or see what else breaks.