APM 2.6 Uncontrollably nose dives!

I was flying my quad-copter in stabilized mode and when i went to bring it back it suddenly nose dives and i can’t regain control, so i flip my switch for rtl ch 7 and it leveled out and hit hard but luckily no major damage. Earlier before this flight i flew perfectly fine and rtl work wonderfully my question is what caused this i have included the log file from this flight. Please Help and Thank You!!

Auto analysis report:

Size (kb) 1424.4677734375
No of lines 19348
Duration 0:09:10
Vehicletype ArduCopter
Firmware Version V3.2
Firmware Hash c8e0f3e1
Hardware Type
Free Mem 0
Skipped Lines 0

Test: Autotune = UNKNOWN - No ATUN log data
Test: Balance/Twist = GOOD -
Test: Brownout = GOOD -
Test: Compass = GOOD - No MAG data, unable to test mag_field interference

Test: Dupe Log Data = GOOD -
Test: Empty = GOOD -
Test: Event/Failsafe = GOOD -
Test: GPS = GOOD -
Test: IMU Mismatch = UNKNOWN - No IMU log data
Test: Parameters = GOOD -
Test: PM = GOOD -
Test: Pitch/Roll = GOOD -
Test: Thrust = FAIL - Avg climb rate 3.49 cm/s for throttle avg 793
Test: VCC = GOOD -

[attachment=0]2015-07-07 19-52-15.bin[/attachment]

Took a quick look at your data and from what I am seeing is that in order for you to hover you require above 70 % input and your mid throttle parameter is set at the default of 500 ( 50% ), at beginning of flight your throttle input was averaging above 700 and towards the end you were in the 800s with a max of 960. Your last commanded RTL utilized all available output of 1000 (100%). You are either under propped, over weight, motor KV is too low or insufficient battery voltage.

 Your multicopter dove more then likely because system did not have adequate power to effectively stabilize; look at it this way, up to 70 % possible power output is for you while the remaining 30% is for flight controller usage to effect stabilization or to carry out pilot commands like turns etc… you are flying in the danger zone which is above 70%

Nil voltage data available from your file because battery monitor is either not active or you don't have one installed thus cant tell how your main battery was behaving

 On a side note, your sytem Vcc was 4.92 max and 4.79 V low; on three occasions your Vcc momentarily dipped as low as 4.73 volts.   Average fluctuations were .13 which is well within limits but those three dips raises an eyebrow.