Is there an abnormal condition that could have caused this fatal accident involving a stall? In 2009, a Grob 103 glider stalled and crashed at Richmond Field, Michigan, killing the passenger but not the pilot. (NTSB accident report: CEN09LA353)

The accident occurred after a failed winch launch; during the winch launch, the tow cable broke while the glider was about 400 feet AGL. The pilot correctly decided to turn around and return to the airport.

Here's what happened next, according to a statement the pilot later gave: 


  I could feel in the controls that something was not right and the glider was not responding in the manner that I am accustomed. There were none of the signs of a stall and the glider did not behave like it has any time that I have practiced stalls. Before I could determine the cause or take any action, the nose abruptly dropped and we dove toward the intended runway.


The NTSB report about the accident states that the probable cause of the accident was simply the pilot's failure to maintain adequate airspeed, resulting in a stall. Judging by this, the NTSB didn't seem to put much stock in the pilot's report that "something was not right".

Suppose that we take the pilot at their word, however. Is there any type of abnormal condition that might have caused a "feeling in the controls that something is not right", and which could cause the nose to drop with "none of the signs of a stall" occurring beforehand?
Your question:


  Suppose that we take the pilot at their word, however. Is there any type of abnormal condition that might have caused a "feeling in the controls that something is not right", and which could cause the nose to drop with "none of the signs of a stall" occurring beforehand?


A couple of thoughts:


In a 60 degree level bank turn, stall speed increases by about 40%. 
See below for information regarding indications of an approaching stall in the G103. (from the G 103 C Twin III Acro manual)





The flight manual, section 4, indicates that with aft cg there is a stall warning 2-3 kts before reaching stall speed - (read) "There is a tail vibration which increases with continuing pull on the control stick." I think the note above (highlighted in yellow) means that with forward CoG, at max weight, there is no stall warning at all, i.e., the glider just stalls (nose drops without warning). I could be wrong, but this seems to me to be what the note means.

Here is a copy of the manual for the Grob G103 Twin III Acro: Grob G103 flight Manual

Here is a copy of the NTSB report I believe covers this accident: Grob 103 Accident - NTSB Identification: CEN09LA353