FLT CREW OF B757 DISCOVER AUTOPLT EXECUTING A COURSE REVERSAL VICE TURNING ON COURSE AT NICKL INTXN WHILE FLYING PRADO DEP FROM ONT.

Narrative

I WAS FLYING THE PRADO 6 DEP FROM ONT ARPT; CA. CLBOUT WAS NORMAL UNTIL MAKING THE L TURN ONTO THE THERMAL TRANSITION. AT THE TIME THE ACFT WHILE ON AUTOPLT; INITIATED THE TURN. INSTEAD OF ROLLING OUT ON THE COURSE TO HEMET AND THERMAL; IT KEPT TURNING L AS IF GOING BACK TOWARDS PDZ VOR (THE OUTBOUND TRACKING COURSE). AS SOON AS I NOTICED THE ERROR; I TOOK IT OFF AUTOPLT AND MADE A STRONG COURSE CORRECTION TOWARDS THE INBOUND COURSE TO HEMET. SHORTLY; THE CTLR NOTICED AND GAVE A MORE STRINGENT HDG OF 130 DEGS OR SO; TO INTERCEPT. AFTER INTERCEPTION; FLT CONTINUED NORMALLY. THIS DEP AT THAT TIME OF DAY WAS EXTREMELY BUSY. THERE WERE MANY INTERMEDIATE LEVELOFFS AND CONSTANT FREQ CHANGES. THAT IS ONE REASON I HAD THE PLANE ON AUTOPLT. TO THE BEST OF MY RECOLLECTION; I THINK WE WERE GETTING ATC INSTRUCTIONS AT THE SAME TIME THE TURN WAS INITIATING WHICH LED TO MOMENTARY DIVERSION ON WATCHING THE TURN AND JUST THE EXTRA SECOND OR TWO ALLOWED THE ACFT TO TURN PAST THE COURSE. WE HAVE NO IDEA WHY THE ACFT DID THIS AS I HAD CHKED THE COURSE IN THE FMC AND IT WAS CORRECT.