  The main asteroid belt (MB) is low in mass but dynamically excited. Here we
propose a new mechanism to excite the MB during the giant planet ('Nice model')
instability, which is expected to have featured repeated close encounters
between Jupiter and one or more ice giants ('Jumping Jupiter' -- JJ). We show
that, when Jupiter temporarily reaches a high enough level of excitation, both
in eccentricity and inclination it induces strong forced vectors of
eccentricity and inclination across the MB region. Because during the JJ
instability Jupiter's orbit `jumps' around, the forced vectors keep changing
both in magnitude and phase throughout the whole MB region. The entire cold
primordial MB is thus excited as a natural outcome of the JJ instability. The
level of such an excitation, however, is typically larger than the current
orbital excitation observed in the MB. We show that the subsequent evolution of
the Solar System is capable of reshaping the resultant over-excited MB to its
present day orbital state, and that a strong mass depletion ($\sim$90$\%$) is
associated to the JJ instability phase and its subsequent evolution throughout
the age of the Solar System.
