Age and lack of maintenance thing pure and simple.
Break fluid will absorb moisture from the air. When it does this it becomes acidic. This then causes several problems.
It lowers the boiling point.
It causes corrosion within the system
It eats the seals.
The rear seal was seeping on the master (possibly a previous failed unit). That fluid sat in the booster and ate at it till it failed.
Older low mile vehicles are very prone to this because its ALL age related. Brake fluid should be changed ever 2-3 years regardless of brake condition. Normally most people will wear out the brakes in that time frame and a quality shop will bleed the brakes as routine in the process of repairing them. A shadetree will often over look this. I'm guilty of it and how I found this out.
Now if you had to replace the modern equivalents your repair bill would have been quadruple if not more if it was a ABS equipped vehicle.
So the rule would be it might not be a bad idea to pull the master off the booster and check for rust and dampness back there on older vehicles.