I've never been aboard this particular vessel, but I've worked on many like it in those waters.
When we operate in open lake waters, the anchors are secured 'in gear' with the brake tightened down, and with claws attached to the chain, and with riding pawls engaged. The odds of it dropping accidentally are vanishingly small. When we're in confined waterways, mostly rivers and harbors, we run with the anchors sitting only on the brake, and maybe the pawl. This is so that we can drop them quickly in an emergency situation.
It's entirely likely that the watch simply forgot to secure the anchors after exiting the St. Marys River a few hours before.
It's also entirely likely that the watch could have been completely unaware of the anchor dropping. That's a large vessel, and the anchor windlass is over 800 feet away from the pilothouse. On my ship, I'm only 500 feet away from the windlass, and I can only just barely feel the anchor dropping, and unless I'm outside on a calm night, I likely won't hear it either. I certainly can't see it. On this vessel, being an articulated tug barge unit, there's no real solid connection between the hull of the barge and the hull of the tug, so there's no way anyone back aft would have felt anything.
Additionally, these tug barge operations generally have a reduced crew compared to ships of similar size. That's part of what makes them economically viable. The upshot is that there may not have been a single crew member aboard the barge to notice. There may only have been two people on watch, one in the pilothouse, and one in the engine room.
The anchors should definitely have been secured, and there's no good excuse for this sort of thing to happen, but it's easy to see how it could have. All in all, it's a bad day for all involved, and will likely continue to be a pretty big headache for a lot of people for a long time to come.
