About a decade ago, I got a text from Peter Gammons. "When did baseball players start having obliques?" Peter knew that players had always had the muscle, but it did seem that we were suddenly seeing more of them injured about that time. This year, it seems to be rib injuries that are the hot new thing in training rooms around the league.
Anatomically, there's no difference. Ribs are ribs, and we have to assume that in almost all cases, MLB players have normal ribs in structure and strength. The game is also much the same—at least in terms of the stresses that it places on the ribs.
We then have to look at the method of injury in context. For Troy Tulowitzki, he was injured on a diving stop of a grounder while at shortstop. Ian Kinsler injured himself twisting away from an inside pitch at the plate, though many point to an awkward headfirst slide earlier in the game as a possible injury point. Pitcher Jake Peavy of the White Sox is a bit tougher to pin down, as he was lifted early but showed poor velocity all through the last start he made before heading to the DL.
One interesting point on all these is that the diagnosis wasn't simple. In each case, an MRI was necessary, and in both Kinsler and Peavy's case, X-rays were negative. Simple rib fractures—the result of collisions—tend to be very visible on X-rays, so that fact tells us that the fractures are either small or in an unusual location.