  Be/X-ray binary systems exhibit both periodic (Type I) X-ray outbursts and
giant (Type II) outbursts, whose origin has remained elusive. We suggest that
Type II X-ray outbursts occur when a highly misaligned decretion disk around
the Be star becomes eccentric, allowing the compact object companion to capture
a large amount of material at periastron. Using 3D smoothed particle
hydrodynamics simulations we model the long term evolution of a representative
Be/X-ray binary system. We find that periodic (Type I) X-ray outbursts occur
when the neutron star is close to periastron for all disk inclinations. Type II
outbursts occur for large misalignment angles and are associated with
eccentricity growth that occurs on a timescale of about 10 orbital periods.
Mass capture from the eccentric decretion disk results in an accretion disk
around the neutron star whose estimated viscous time is long enough to explain
the extended duration of Type II outbursts. Previous studies suggested that the
outbursts are caused by a warped disk but our results suggest that this is not
sufficient, the disk must be both highly misaligned and eccentric to initiate a
Type II accretion event.
