Sounds like a setting in the firmware, or maybe a something with the board passing the raw signal through to the motors while powering down. can you give the link to the exact ESC?
Programming Functions: Battery Type: Lipo /NiXX Brake: On / Off Voltage Protection: Low / Mid / High Protection mode: Reduce power / Cut off power Timing: Auto / High / Low Startup: Fast / Normal / Soft PWM Frequency: 8k / 16k Helicopter mode: Off / 5sec / 15sec (Start up delay)
Could be bad, may try contacting HK to see if they'll send you a new one, or if they have had this problem before. (providing you got them form hobbyking.com)
Hi sorry i was not about was at work , look at the transmitter lower the throttle curve to give you a neg number , I had this on my 500 heli every now and them the rotor will turn so now throttle is -5 this will stop it , some ESC can play up