I believe the 319/769 has the same GEC traction motors as the 317, so it might be expected that ageing of the enamel insulation would afflict both classes equally.
However, the traction control electronics are different. The 317 is an AC only unit and has thyristor control, with the thyristors commutated by the AC supply. This was unsuitable for the 319 dual voltage unit, which was the first BR application of an Insulated Gate Bipolar Transistor (IGBT) chopper control. Earlier DC EMUs had electromechanical controls.
This is speculation on my part, but it is theoretically possible that the 319 control allows the motors to be stressed by a higher maximum current on startup than the 317 control, thereby increasing the risk of failure.
Rewinding a burned out motor is a costly repair, and I understand Northern experienced a number of failures before mandating a more cautious driving policy across its 319 fleet.