Water problem with Oak engineered floor

We got an oak engineered floor (clipped together not glued) laid on our concrete dining room floor last year. A few months ago, I noticed black discolourization at one of the joints between two planks. This increased over time and soon other planks became affected.

The area affected is near the patio doors, but there was a laminate floor and previously carpet in the area with no problems. House is seventeen years old.

We had someone take up the planks today and some of them are sodden and beginning to rot. The guy could not find any source for the water as the concrete was dry.
We got a new kitchen (+ diswasher) put in last year. That is the only other change I can think of. It is your standard kitchen come dining room with tiles in kitchen area.

Anyone have any ideas at what might be causing this? We are at a loss. We do have a dog that does the odd piddle, but she doesn't necessarily stick to this section of the floor and there are no problems elsewhere.

Thanks,
Maggs

We have the same floors and seem to be laid the same way, i.e. ontop of concrete in dining area of kitchen.. noticed near the external door there is a very very slight black discoloration, but nothing to be alaramed by.. i am wondering if it could be moistness from the door (which is pvc)..

We have discovered it is the dog pee that is causing the discolourisation! She is left in the house overnight and the pee can be sitting on the floor for some hours before we get to it. We had a guy in to take up the effected boards and alot of them had started to rot underneath.
The plywood base is particularly susceptible to water.

Our expert said to be careful even when washing the floor and to use a well squeezed mop! So doggie is to be moved to utility where hopefully the ceramic floors won't be affected by her nocturnal expulsions!

good too know,thanks for the update.