It's a known issue, a conflict with KDE's Qt-4 libraries. Editing the <spine> section of content.opf, saving, and reloading is the workaround.
With the 0.7.x series, which uses Qt5, the conflict is avoided (at least until KDE moves to Qt5). Moving chapters around via dragging works.
As an afterthought, if you want to upgrade to Sigil 0.7.3 without having to install the apppropriate Qt-5x package and build Sigil from source, you might consider DiapDealer's
"Unofficial Sigil debian packages."
I'm using the 0.7.2 package on Kubuntu Raring (13.04) successfully.
HTH
Albert