From what I've been reading on this testosterone and steroid thing, the soft muscle tissue grows and becomes stronger before the connective tissue (ligament, tendons) can have a chance to adjust (either strengthen, increase flexibility or both). Sorry about no links. The info is out there but it might be hard to find. Essentially, the stronger muscles break the weaker components.
There's also some discussion of a training effect ref. epo use, that the increased rbc overtime create a greater density within the capilary beds. Also, cardiac output might also increase over time due to the greater workload associated with pumping blood with a greater viscosity and running a higher heart rate for long periods before reaching LT. Athletes who use epo run the risk of wearing prematurely different structural components of their hearts, like valves, for the same reason.
My theory: Giambi and Bonds increased their weight and strength beyond the point that their skeletal system and connective tissue could keep up. It's like when you rebuild the engine of a car. Not soon after, the new engine will wear out the old transmission. So basically, they were just tearing themselves apart. Bonds, it would seem, was using HGH, which should also grow everything (most noticably his head) but it grows soft tissue faster. I'd think none of this applies to cyclists since it's not a weight bearing sport. However -- and I don't understand exactly how this works -- epo causes a weakening of the bones, or so I've read. So crashes can be pretty disastrous -- broken hips, femurs.