The purpose of using ping pong balls on top of a Sous Vide Bath

Pretty much...yes, but you can fix it!.

When you properly sous vide or very slow cook anything, you'll retain more of the myoglobin color because of the even cooking that often doesn't go above 140 at all. So a properly cooked steak like this will retain much more of its red colored myoglobin. Simply put, the meat will have more red juices to release! (Its a great, great thing about sous vide.)

As @Ronald mentions, the other thing is the resting of the meat. It's an important step as the muscle fibers relax after the heat is off and hold juices better then. An often misstep for the home cook is they allow a hot piece of meat to rest on a flat, solid surface. This causes the bottom of the meat to steam against the board, open the fibers in the meat more, and release the juices on to the board. Rest your meat on a raised baking rack so that it has air circulation all around it.

After a short rest - for most steak 10 minutes is fine, then you can cut into the steak. Use a very sharp knife to slice. The meat here is essentially a sponge and you don't want to compress it and squeeze out the juices. A dull knife will do this and you'll lose more juice on the cutting board again. Use a sharp knife and apply steady, even, but light pressure while slicing - let the edge do the work (if it won't, sharpen the knife more).

Anything you salt will firm up in texture over a period of time. I suspect that since you cooked these with seasonings and then chilled and left them in the fridge before reheating an eating they firmed up a great deal in the fridge.

If you check out this blind tasting conducted by Dave Arnold at Cooking Issues you'll find some more detailed info about this topic. The gist of it is that for cook-chill-reheat purposes you shouldn't salt the meat before searing. For cook-direct serve meals your just fine doing it that way.

http://www.cookingissues.com/2011/10/12/to-salt-or-not-to-salt-thats-the-searing-question/