A better question is "WHy is the Pioneer probe in, presumably, Klingon Space?"
Early Federation, Pre-Federation, even Pre-Starfleet space tech always has a way of ending up in places it shouldn't be. Nomad, Pioneer, V'ger, Ares IV, Friendship One, Charybdis, the Essex, the Valiant... and I'm probably forgetting a few, too. Enough examples to qualify it as a trope.
...none of the others screamed
, though. Metal stress? I don't know.