Unexplained EKF failure

I took my pixhawk for it’s first flight and performed fantastic until, for some reason, EKF_CHECK-2 error occurred causing hard forward thrust. I need help analyzing the attached log. I spent the first few days learning the log utility in Mission Planner and reviewing the log and am blown away as to how much information it logs. Very impressive. But when the EKF_CHECK-2 occurred, it did a hard forward on it’s own then i threw it in to RTL. The second time it occurred (same flight), it nose dived in to the ground (luckily. better than flying away.) gps was spot-on for the duration from what the logs says. I just can’t wrap my head around the EKF failures and how I should interpret and what I should do. Logs attached.

Much appreciated in advanced.

I am going to move you the to the ArduCopter v3.2.1 sub form as this is not necessarily a hardware issue.
Here is an Auto Analysis of your dataflash log:

Size (kb) 3589.4453125
No of lines 49871
Duration 0:03:30
Vehicletype ArduCopter
Firmware Version V3.2.1
Firmware Hash 36b405fb
Hardware Type PX4
Free Mem 50800
Skipped Lines 1

Test: Autotune = UNKNOWN - No ATUN log data
Test: Balance/Twist = GOOD -
Test: Brownout = GOOD -
Test: Compass = GOOD - mag_field interference within limits (8.09%)

Test: Dupe Log Data = GOOD -
Test: Empty = GOOD -
Test: Event/Failsafe = FAIL - ERRs found: CRASH FENCE
Test: GPS = GOOD -
Test: IMU Mismatch = GOOD - (Mismatch: 0.75, WARN: 0.75, FAIL: 1.50)
Test: Parameters = GOOD -
Test: PM = FAIL - 20 slow loop lines found, max 8.88% on line 28341
Test: Pitch/Roll = GOOD -
Test: Thrust = GOOD -
Test: VCC = GOOD -