  (ABRIDGED)- The physical mechanism responsible for the short outbursts in a
recently recognized class of High Mass X-ray Binaries, the Supergiant Fast
X-ray Transients (SFXTs), is still unknown. Two main hypotheses have been
proposed to date: the sudden accretion by the compact object of small ejections
originating in a clumpy wind from the supergiant donor, or outbursts produced
at (or near) the periastron passage in wide and eccentric orbits, in order to
explain the low (1E32 erg/s) quiescent emission.Neither proposed mechanisms
seem to explain the whole phenomenology of these sources. Here we propose a new
explanation for the outburst mechanism, based on new X-ray observations of the
unique SFXT known to display periodic outbursts, IGRJ11215-5952. We performed
three Target of Opportunity observations with Swift, XMM-Newton and INTEGRAL at
the time of the fifth outburst, expected on 2007 February 9. Swift observations
of the February 2007 outburst have been reported elsewhere. Another ToO with
Swift was performed in July 2007, in order to monitor the supposed ``apastron''
passage. A second unexpected outburst was discovered on 2007 July 24, after
about 165 days from the February 2007 outburst. The new X-ray observations
allow us to propose an alternative hypothesis for the outburst mechanism in
SFXTs, linked to the possible presence of a second wind component,in the form
of an equatorial disk from the supergiant donor. We discuss the applicability
of the model to the short outburst durations of all other SFXTs, where a clear
periodicity in the outbursts has not been found yet. The new outburst from
IGRJ11215-5952 observed in July suggests that the true orbital period is
~165days, instead of 329days, as previously thought.
