You could get I-131 from a warhead fizzling. I don't know exactly how weak a fizzle it would need to be in order to escape seismic detection and whether that would be large enough to produce the measured amount of I-131 though.
If a warhead fizzled, it would have destroyed the submarine carrying it, which would have detonated hundreds of tons of rocket fuel and the rest of the explosives on board. We would have known about the explosion the same way we did the Kursk.
Whoops launched the nuclear torpedo instead of the training torpedo good thing the self destruct routine is an intentional near zero yield fizzle. My guess is the board and court martial would so classified we wouldn't hear about it for a century.
That would be a very strange design choice, to have the warhead self-destruct in that way. I don't know how much is really around in the open literature on ex-Soviet / Russian nuclear torpedoes, but certainly there's nothing around to suggest that US nuclear weapons do that, for a variety of good reasons, and I don't think there's any reason to suspect that the Russians designed theirs much differently. Off the top of my head: first, most nuclear weapons don't have a self-destruct or recall capability once launched, because that's a vulnerability during actual use; two, if it were launched unarmed, then the expected result would be a failure of the actual detonation system, leaving an intact weapon somewhere at the bottom of the ocean; third, a fizzle of a nuclear weapon underwater would probably cause a substantial amount of contamination from the fissile materials; it would basically be a (very) "dirty bomb", and difficult to hide. We'd see significant long-lived fissile contamination in the water either already or very shortly.
The much more likely explanation is that a reactor, either onboard a ship or submarine or on shore, had some sort of (probably minor, in the sense of "not catastrophic") mishap and vented steam from a primary cooling loop. You would expect to see short-lived byproducts including Iodine-131 in this case. There have been a number of similar accidents over time, and a release of steam wouldn't necessarily have any other signs and it might be tempting to cover up if it occurred at sea.
