Bunk trailer? Winch strap or cable?
On steepish ramps, the rear of the trailer is lower in the water than the front and more importantly the front of the bunks(assuming bunks). If roller, the same thing happens but the distribution of weight depends on roller positioning and setup.
In either case, the initial load of the hull is more towards the middle of the hull since the aft is still floating. As the boat is hauled less of the rear is floating and the weight shifts aft causing the hull to tip and slip backwards and putting a lot of strain on the winch. Yes the strap or cable can stretch. The strap will usually stretch more.
If you can, haul the boat a little way up the ramp and re-tighten the winch..
For rollers it is easier to just haul the boat and as mentioned use the start and stop method to roll the hull forward. Bunks may no slide that easily and I don't like putting all that rubbing force on the hull especially if painted.
If bunks, installing that slippery plastic bunk material make help but remember, it works in both directions
Don't assume the hull will not move if you loosen or undo the winch