What van is yours based on? Or Chassis?
Is it an L4, as they have the heavy duty suspension.
But I'm more and more thinking that for some reason it's a weight thing.
I don't know what your kerb weight is (van, you, oils, coolant, fuel, and all other bits & bobs you carry,
But
I guess when you've a car on the back, it's mass weight will be higher than say if I had the equivalent weight on the floor of my van, so when you brake, that higher mass creates a 'g force' effectively on the front suspension that's greater than if lower in a van. Pushing down on the front suspension more than if it was lower than a van, I really don't think the strut mounts on your car transporter are up to the job, as they probably are not designed to take that regular heavy load especially when braking.
Let's say you chuck an Audi A4 Estate on the back
That's just under 1.7 tonne kerb weight. That's rocking higher than sat on the chassis (in a van spread over the front and rear suspension as its lower)
If I put 1.7 tonne on my L4 van I would be 700kg overweight
My kerb weight is just under 2.5tonne
I would look at upgrading if poss the suspension to at least what the ambulance's have (same as my old 3.0 Euro 5 L4 H2) if it hasn't got this already, those ambulances carry some weight