I've not had any decoupling issues with my DODX cars, but I did have to grind the wheel wells to eliminate screeching. Interestingly, that issue only occurred on my cascade green flats.
I do have a set of Intermountain 60' flatcars with MTL couplers that uncouple if you look at them funny. I've pinpointed that issue to couplers not properly centering. More specifically, the knuckle will swing left (as your facing the end of the car), but fail to return to center. If the pull on that coupler becomes zero or negative at any point, that is when the decouple happens.
Why wont the Intermountain couplers center? For some reason they do NOT have those little nubs
@Missaberoad mentioned above. Each half of the coupler should have a nub, and as the spring pushes the coupler away from the screw/pin, those nubs catch in a little hole/channel on either side of the coupler box, which pushes them into each other, both closing and centering the coupler.
As you face the end of an upright car, the channel on the bottom side of the box returns the coupler to the left, the channel on the top of the box returns the coupler to the right. The only part I haven't understood is why the wing of the coupler is not preventing the decouple, as it should be keeping the knuckles sort of locked, even if the coupler is swung to either side. This may be a separate issue with a weak spring.
Ok, back to the DODX cars. The bottom lid of the coupler box is a press fit piece. There is a center post and either side is notched to fit and align into the rest of the coupler box. Quite clever design I think, but it is made of plastic and can fairly easily be 'overruled' and fit together out of alignment.
I'm looking at the bottom of the car now, holding so that the coupler is pointed up. The bottom lid on the coupler box can be easily rotated in either direction, and kind of acts like a 'trim' adjustment to the center of the coupler! It's not supposed to do that, but it does. So if I rotate that part clockwise, by no more than 2 or 3 degrees, the center of the coupler also moves to the right (more 'closed'). Likewise, if I rotate the part counter-clockwise, the center of the coupler moves to the left (more 'open'). It doesn't seem like much, but I'd wager if you inspect the cars that uncoupler, those couplers are more than likely 'trimmed' left.
So, with all of that, my suggestion for a quick fix, try 'trimming' the couplers to the right. If that doesn't work, a more involved/permanent fix could be to lengthen the channel on the bottom of the coupler box so that the coupler is allowed even more travel to the right (closed) position. But you might want to connect with MTL for their ideas before performing destructive surgery.
