I think that comes under the heading of sufficient stimulus.
I seem to remember us discussing that issue at the time and I put it down to lack of proper maintenance. From what I remember it turned out that you hadn't been wiping out the inside leather lining with a damp cloth on a regular basis to any remove sweat/salt/dust build up and this eventually lead to the leather lining hardening and splitting causing your problem. You also hadn't put any Berghaus leather conditioning cream (or similar leather conditioner) on the leather lining to keep it soft and pliable and in good condition as Berghaus recommend.
I have personally found that Nikwax boot proofing products do not keep boots waterproof for very long at all and seem to just wash away with any prolonged contact with water.