I will explain this once and then walk away from the shitstorm it will cause not to return.
If you cross your straps, you only have to lose or have 1 strap loosen in order for the rest to also be loose. Pulling on things at an angle also reduces their working load considerably. Put another way, the force they can exert to keep the vehicle from moving front to back is reduced considerable since the vehicle only has to move a small amount for them to be loose.
If you take the straps straight forward and back to conveniently placed tie down points, you can lose two straps and the other two will still be tight. You can loosen two and the other two will still be tight. Keeping them tight is key to them being able to do their job.