Why do my straps do this, and how to stop it?

I think is from the bend in the hook. It's not as tight as your old ones.

Can you lay them down on the deck side by side and compare?

 

why not hook them under the trailer, i don't pull flatbed anymore but when i did we used to hook under the trailer.

 

Put the hook back down thru the rail and see how it tightens .

 

I don't even wrap the hook. Just drop it down the inside and hook it on the bottom of the rail... Although, Its a PITA when its windy.

 

Why are you hooking to the rub rail? Its there to protect your straps... Not to hook them to.

I agree with @Grumppy ... The way you are hooking them, the strap lifts the hook.

 

Here's a tip. If you want to use the rub rail then wrap it back through one more time so the strap will be pulling the throat of the hook into the rail. Otherwise as others have said hook it under the deck.

 

Like everyone else is saying, hook it under the lip of the deck, that's a reitenour from the look of the stake pocket extrusion, that rub rail isnt supposed to be hooked to

 

Some states, Mississippi for one, will write up unsecured load for being strapped to rub railand make you restrap it.
Seen a guy at scale house get a write up while my permit was being checked