Ever wondered whats behind the upper rot???

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The rusting is a "feature" of galvanised steel multiplate body shells.
It is not caused by large volumes of water just condensation in a closed body section.
Any place where two plates are joined can form a construction trap.
Position and shape of the plates can be critical hence some locations very prone to this problem.
Fabrication can damage the protective coat in the local area.
The thin space between the plates sucks in air and water by capilliary action.
Optimum rust conditions are oxygen and water in equal volumes.
Steel with damaged galvanising suffers from accelerated rusting at the damage points.
Age and usage of the car are not a guide to the rust risk.
It is more about where the cars has spent it stationary life causing varied amounts of condensation in the closed members.
Because the rusting is from the inside outwards it comes as a nasty surprise when it breaks through.

John