All this is conjecture but it looks to me like the big tug was pulling the sail ship and the smaller tug up river after a failed attempt to get through the bridge, or maybe it got through and then the small tug got sideways somehow. The smaller tug over on it's side so the big tug let it all loose. The smaller tug then righted itself (they are massively ballasted and can roll over and still right themselves) after that, it's just the small tug trying to keep the sail ship off the bridge. It can't do that and then backs away to get distance from the sail ship as it starts capsizing under the bridge. It looks like the sail ship is prevented from fully rolling over by it's yardarms which get caught up on the bridge. After that the big tug is seen returning to hook up and pull the sail ship away. All this an attempt to get the sail ship through the opened bridge ( either going upriver or down) that went awry when the whole works got misaligned by the current.

Again, I'm just guessing.