very smart of you to use the safety bars--------too many assholes dont take 30 seconds out of their life to set them up and pay for it for the rest of their lives
sometimes a muscle can cramp or some little thing happens and your legs give out------its common, I had it happen to me on the smith machine while squatting years ago and lucky I had the safeties set up, next set i pumped out 10 reps of the same weight, I have no idea what happened that first set but thanked god I set up the safeties.
this should be a lesson to all the young lifters out there