Why would a player who came into a match with injury be found to retire consistently when behind? If it is always due to injury, or injury-proneness, then you should see that it happens to a player as much when he is ahead as when is behind. Sure, you can argue that the injury aggravates during a match he is behind in, so that he has to retire. It can happen, 3, 4, 5 times. But when it happens 11 times that you retire when you are behind, there seems really nothing suspect about the data to me.
You have an injury, you had a few chances to win despite it but weren't able to capitalize. And now you have like 2 games before the match is over in which to equalize and overtake the opponent. What should a reasonable player who does not want to risk aggravation with no benefit in sight (of getting ahead) do?
Because these guys are competitive, they want to play if they at all can, so they might play a match, like Ljubicic did against Nalby, just in case they might be able to win. And then at the point they get far down, that's when they realize they can't, or maybe it's coincidental that that's when they aggravate the injury? Like Ljubicic said, if he had gone down a break in the 2nd, he would have retired, but he felt like as long as he could keep hanging on that he'd continue to play. He managed to win. I know that Andy has done the same thing (when he beat Ferrer saving MPs in Paris a couple years ago comes to mind)
I just have a problem with the fact that they are lumping all these kinds of retirements together to make generalizations about it. A guy who comes into the match with a perhaps healing-but-not-quite-completely-healed injury who retires down 1-5 in the first when he realizes that the ankle is not going to make it through the end of the match is going to be lumped into the same category as a guy who retires down 1-6 1-4, at least that's how it seems from the explanation... and i have a problem with that.
Just for an example of one i can think of at the top of my head. You have Andy who hurt his ankle badly at Dusseldorf. he does everything in his power to try to play RG, but he goes down 2 sets and a break and can barely move and sits down talking to the trainer crying, and decides to retire. Do you really think he retired b/c he's a bad sport, or because the trainer tells him he could injure his ankle even further and he knows damn well there's no way he can't come back to win with a bum ankle (mind you, he reinjured it late in the year and DID finish the match where he reinjured it, but was out the next 2 weeks
It's the generalizations and ignoring so many relevant facts that bother me.