How does pineapple tenderize meat?
Update: After reading a bit on this, (and also fully reading the question) I found that there have been papers written on the effects of acid baths on meat tenderness as well as enzyme treatment: 
Acid: http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1010&amp;context=animalscidiss
Enzyme: http://www.brainromania.ro/uploads/papers/effect-of-marination-with-proteolytic-enzymes-on-quality-of-beef-muscle-2012-409.pdf

It seems that food grade citric acid doesn't affect tenderness that much, where as a solution of Bromalain does, and at a greater effect than that found mixed in with the citric acid of Pineapple. There's no study comparing pineapples to oranges (Eh? See what I did there?) but this seems to say that the citric acid in food may have less of an effect than that of the digestive enzymes in them. 

Please see below for previous answer if you'd like to point and laugh at those who skim questions and don't read fully.  

So, I think I found the answer: http://science.howstuffworks.com/innovation/edible-innovations/pineapple-enzyme-tenderize-steak.htm

Apparently, acidity aside, the enzymes found in the stem of the Pineapple: Bromelain can divide the proteins found in muscular fiber. I'm going to copy and paste the next part from the article that goes a bit deeper into how Bromelain works (and I'm lazy):


  Bromelain works in these capacities due to it's ability to separate amino acids. Amino acids are organic compounds within living cells. Amino acids join by forming peptide bonds, a link that connects one amino acid's amino group with the carboxyl group of another amino acid. When amino acids join through peptide bonds, they form proteins. These proteins can carry out numerous functions in the structure and operation of cells, tissues and organs.


This breaks down the peptide bonds that hold together proteins in collagen, which is what makes meat tough. The enzymes stop around 158 degrees Fahrenheit, so once they start cooking, their work is done. That's why it's important to marinate.

TLDR: Pineapple stems have Bromelain, which aside from a fun nickname breaks down Amino acids and dissolve the peptide bonds in collagen, which causes the firmness in meat.