Falling off a horse and slipping on ice
Bodybuilders are notoriously fragile. Being very strong might also have to do with it, not just weak tendons from steroids per se, slipping on ice the leg extension will be very powerful lol. Though it's not that uncommon for regular folks to tear something doing something silly either, like tearing a bicep lifting a sofa, washing machine or fridge. Track & Field athlete tearing a calf isn't unusual at all

My boss looked at me unapprovingly when I tore a bicep, like it was my fault so to speak, "what, that never happens just lifting something like that!" What?

When WSM Magnus Samuelsson broke a competitors arm while armwrestling it was because Magnus is so strong... when it could be because the other guy is so strong as well, too strong for his bones lol.