Could be from over speeding. Those entry level 'inverted' style engines are built for price, materials and tolerances are suspect.

When I was in the business, we would call that
"Bad Broke"

no big deal remove the jug piston/connecting rod from donated engine and crank it up. Fix them all the time, if I have the same model engine I just remove the piston/connecting rod and install one out of another engine. Usually they start loosing needle bearings. I found them in mufflers or falling out on the ground. Especically 31CC craftsmans with the spark plug on the bottom. As far as why it broke? unless you know the complete history could be material flaw that finely showed up?

I like rcmoser idea of parting these trimmers back together if you have the needed spare parts. I stripped an old commercial Robin #NBO2T for its straight shaft (and some screws) with a drive rod vs a cable. I spared the other trimmer since it appears to only need another connector rod. It also appeared easy to replace the rod with the jug/case cast as one with a side cover to access the needed repair. I'll give it to some person or shop that has parts for that model. It was two experiences, # 1 to see the jug/case cast as one, # 2 to see the broken connector rod w/o other damage. Good luck With Some Experiments/Cleaning/Clearing!