Since the OP’s observations suggest that some folk might benefit from a bit of advice…here’s my two penne’orth
When tightening straps, tighten them in the following order:
Straps which will keep the load secure when accelerating
Straps which will keep the load secure when braking
Straps which will keep the load secure when cornering
Secondly, when you have loaded your vehicle, drive it away keeping the load under observation as best you can, then stop after approximately five miles and re-check your straps.
Thereafter whenever you stop and take a break, take that opportunity to re-check your straps.
Hope that’s helpfu!