BEFORE DEPARTING, THE PLT OBTAINED A WX BRIEFING & WAS ADVISED OF A SIGMET WHICH FORECASTED SEVERE TURBULENCE BELOW 18,000 FT, STRONG UP/DOWNDRAFTS, STRONG SURFACE WINDS & WIND SHEAR NEAR MOUNTAINS & IN PASSES. HE TOOK OFF WITH FULL FUEL TANKS & APRX 400 LBS OF CARGO ON BOARD. ARTCC REPORTED THAT THE EN ROUTE PORTION OF THE FLT WAS HANDLED ROUTINELY. DURING ARRIVAL, THE PLT CONTACTED THE ARPT UNICOM FOR A LANDING ADVISORY. A KING AIR PLT WAS ARRIVING AT ABOUT THE SAME TIME & ASKED THE AEROSTAR PLT IF HE WAS MAKING A FULL STOP. THE AEROSTAR PLT REPLIED THAT HE WAS. THE KING AIR PLT LOST SIGHT OF THE AEROSTAR & BEGAN A 360 DEG TURN. HE ASKED THE AEROSTAR PLT TO ADVISE OF HIS POSITION, & SHORTLY THEREAFTER,THE AEROSTAR PLT TRANSMITTED 3 MAYDAY CALLS. REPORTEDLY, THE ACFT NOSED UP & WENT INTO A STEEP RGT BANK JUST BFR IMPACT.NO PREIMPACT MECHANICAL FAILURE/MALFUNCTION WAS FOUND. THERE WAS EVIDENCE THAT MOST OF THE CARGO WAS IN THE BAGGAGE COMPARTMENT (LIMITED TO 240 LBS) & COULD HAVE MOVED THE CG WELL BEYOND THE AFT LIMIT. ELEV TRIM FND FULL NOSE DOWN.