Chicken marinade with soy sauce - have I done something wrong?
Soy sauce is pretty salty. It sounds like a great deal of water diffused out of your chicken and into the marinade, which significantly changed the texture of the meat.

It's not uncommon to do something like this on purpose. When you make gravlax, for example, you cover a piece of salmon with quite a bit of salt and refrigerate it for a day or two. The salt draws out a lot of moisture, causing the fish to firm up considerably. Even though your soy sauce marinade was liquid, it still had a much higher salt concentration (or to think of it another way, a much lower water concentration) than the chicken, and would have had the same effect.