THE PLT WAS ON A X-COUNTRY ACCOMPANIED BY A PLT IN ANOTHER ULTRALIGHT VEHICLE. THE ACCIDENT ACFT WAS FASTER, SO THE PLT MANEUVERED AROUND WHILE EN ROUTE. DURING A TURN, THE ACCIDENT VEHICLE GOT OUT OF SIGHT FROM THE PLT OF THE OTHER VEHICLE & BECAME PERMANENTLY SEPARATED. AT 0957 EDT, A BOEING 727 CREW REPORTED A NEAR MISS WITH 2 ULTRALIGHTS IN THAT VICINITY. THE CAPTAIN ESTIMATED A SEPARATION OF APRX 100 FT. HOWEVER, HE OBSERVED THE ULTRALIGHTS AGAIN ABOUT 10 TO 15 SEC LATER & SAW NO EVIDENCE OF AN UPSET. AT ABOUT 1000 EDT, WITNESSES SAW THE LEFT WING OF THE ACCIDENT ACFT FOLD UPWARD, THEN HEARD A LOUD POP & SAW THE ULTRALIGHT FALL VERTICALLY TO THE GROUND. AN EXAM OF THE WRECKAGE REVEALED ONLY OVERLOAD FAILURES. REPORTEDLY, THE ULTRALIGHT WAS INVOLVED IN SOME TYPE OF MISHAP ON THE PREVIOUS DAY. THE CIRCUMSTANCES WERE NOT VERIFIED, BUT THE PROP & THE AXLE OF THE MAIN WHEELS REQUIRED REPLACING. DURING CONSTRUCTION, THEBUILDER HAD MADE MODIFICATIONS TO THE ACFT.