Insurance claim

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How does this work with a load-shifts?

We had a scenario where 2 different drivers at two different days trucks and trailers hauled the same shipper's freight and at the delivery both had shifted cases.

We had to cover the re-work fee, but technically speaking how can we prove that it was shipper's fault, incorrect loading, if the driver was not allowed at the dock?
Load should be loaded so that it can withstand hard braking and an abrupt swerve. Driver may need to hard brake or swerve to avoid someone else’s mistake. A shifted load indicates improper securement.. whoever loaded it is responsible and should be held accountable.