How the ever-living shit does something like this even happen?
Steroids, that’s how.
True it makes it much more likely. However, and I haven't checked data, biceps tears don't seem that uncommon in the general population. I was surprised to hear that many people knew someone who had this happen. Regular folks. Most aren't even recommended surgery even with a full tear which is what I had, I was discouraged too.
I tore mine lifting a welding machine with my moddle finger, there was like a loop at the bottom, over a door sill. And I've been paranoid about this exact injury for like 25 years.
Achilles tears don't seem that uncommon either, everyone seems to know someone.
4 weeks after the operation I was lifting 2.5 lbs plates to start with that arm.
My surgeon was mad because I was rehabbing with too light weights, said at 12 weeks post op I was supposed to be back at full strength. "But I thought I was supposed to be careful?" "No we are not aiming at getting back some function, we are aiming at getting back full strength. As long as you don't yank the weights you are fine." Now at 2 years post I'm not back at full strength still, but that's just because I haven't been training enough.