I may have missed it in one of the pictures, but do you have feet on your ramps somehow or do you have flip down feet attached to the rear of the trailer? It's important.
When I borrowed a homemade trailer years ago when moving my old cars, it only had two thick wooden boards for ramps. I had my truck hitched to the trailer, but the whole rig was on a very slight downhill slope. I was loading my old 64 impala, and as soon as the front tires got to the back of the trailer, the weight of the car had pushed down on the back of the trailer enough that it lifted the rear of the truck off the ground and the whole shooting match started drifting down the hill. I panicked and stopped the impala, the trailer and the truck kept going forward till the ramps had pulled themselves out from underneath the car and the truck dropped back down and stopped. I had to get my wife outside to hold the brake in the truck before I could load it.
On the trailer I built, I have very light metal for the ramps, so I put a truss under the ramp like was pictured in the beginning of this thread. I didn't put any legs at the back, but when weight gets on the ramps, the truss comes down and hits the ground and keeps everything stationary and keeps the truck on the ground.
That's one thing you still have to watch for though. We were loading a 7000lb skid steer on my trailer a couple of years ago, and I mistakenly parked the truck and the trailer on a hump. That made the back of the trailer very high off the ground. As the guy was loading the skidsteer, the trusses had a long way to go before they hit the ground. I looked at the truck as he was coming up the ramps, and the tires of my f250 were at least 6 inches off the ground till he got the skidsteer all the way up on the trailer.