Closely check the dropout width to the axle and/or nut; if this is worn wide it decreases the contact area for it to butt up on and hold, This is why some nuts have flanges with ridges, or ridged washers are used.
I have also seen lengthwise wear on slots (such as the dropout) where it gets thinner in the load direction (forward, in your case), which, with decreased contact, allows the assembly to shift.