My Dash 8 tech is getting a little hazy now, but doesn't the Dash 8 AFx system look simply to the torque gauge output to determine if an engine has failed?
Therefore in the event of a torque gauge failure (ie loss of power) during take-off the AFx may interpret the nil output from the gauge as an engine power loss - and hence feather the prop...
This happened to a skytrans -100 in Darwin during my time there and I recall that the above was something like what caused it. Other aircraft types use oil pressure loss (from a dedicated transducer) for their AFx, which is probably a more reliable determinant of engine failure.