You were likely torquing it bigtime. I have seen this in youth or folks with a draw wt that is a bit too much.
One other possibility is that you had something in the cam, stick, grass, etc...

Letting a bow down is usually not anything like firing it. The more twisting or holding it away from you the more likely this can happen. I would also venture to assume that you have the bow turned down to the low end of the draw wt. or below what it is made for. This causes slack when starting forward in the buss cable and/or string and could also be a cause of this. It is not a good idea to have a bow at too low poundage.

Did you get it back on?..... If so, it should be OK, just be sure all the slack is out that you can get out. I realize you have draw wt issues.
Also be sure not to torque the bow when letting it up. Torqueing is result of gripping too hard with your bowhand.