  We investigate the properties of a variability-selected complete sample of
AGN in order to identify the mechanisms which cause large amplitude X-ray
variability on time scales of years. A complete sample of 24 sources was
constructed, from AGN which changed their soft X-ray luminosity by more than
one order of magnitude over 5--20 years between ROSAT observations and the XMM
Slew Survey. Follow-up observations were obtained with the Swift satellite.
After removal of two probable spurious sources, we find that the sample has
global properties which differ little from a non-varying control sample drawn
from the wider XMM-Slew/ROSAT/Veron sample of all secure AGN detections. A wide
range of AGN types are represented in the varying sample. The black hole mass
distributions for the varying and non-varying sample are not significantly
different. This suggests that long timescale variability is not strongly
affected by black hole mass. There is marginal evidence that the variable
sources have a lower redshift (2$\sigma$) and X-ray luminosity (1.7$\sigma$).
Apart from two radio-loud sources, the sample have normal optical-X-ray ratios
($\alpha_{\rm OX}$) when at their peak but are X-ray weak during their lowest
flux measurements. Drawing on our results and other studies, we are able to
identify a variety of variability mechanisms at play: tidal disruption events,
jet activity, changes in absorption, thermal emission from the inner accretion
disc, and variable accretion disc reflection. Little evidence for strong
absorption is seen in the majority of the sample and single-component
absorption can be excluded as the mechanism for most sources.
