  The majority of magnetic chemically peculiar (mCP) stars exhibit periodic
light, magnetic, radio, and spectroscopic variations that can be adequately
modelled as a rigidly-rotating main-sequence star with persistent surface
structures. Nevertheless, there is a small sample of diverse mCP stars whose
rotation periods vary on timescales of decades while the shapes of their phase
curves remain unchanged. Alternating period increases and decreases have been
suspected in the hot CP stars CU Vir and V901 Ori, while rotation in the
moderately cool star BS Cir has been decelerating. These examples bring new
insight into this theoretically unpredicted phenomenon. We discuss possible
causes of such behaviour and propose that dynamic interactions between a thin,
outer, magnetically-confined envelope braked by the stellar wind, and an inner
faster-rotating stellar body are able to explain the observed rotational
variability
