Arch-shaped magnetic flux tubes are generated in a pulsed laboratory experiment. As the axial plasma current rises, the apex of the arches is found to expand with constant velocity. This conflicts with the common assumption that plasma expansion in this type of configuration is caused by the hoop force. We propose that drift movement in the electric field arising from the experimentally applied voltage pulse and the plasma current's magnetic field can lead to a significant alteration of the expansion characteristics. To this end, probe measurements of magnetic and electric fields in the plasma are presented and the corresponding drift velocity is evaluated. The proposed mechanism is discussed in the context of recent results of numerical (Tacke et al 2013 Phys. Plasmas20 072104) and experimental (Stenson et al 2012 Phys. Rev. Lett.109 075001) investigations of flux rope expansion in similar configurations.