My guess on what happened
Since there are two boats, it may not be as simple as one guy completely oblivious. The boats had probably seen the bridge open and boats before them go under the open bridge. The may have even called the bridge tender and let them know that they were coming. They were probably a little further away and once the bridge tender saw a big gap between the boats for them and then nothing, he thought everyone was through and the bridge closing sequence was started. Not all bridges have alarms and lights for the boaters. Or, they may have seen the open bridge, thought it was going to stay open for them, thought bridge tender knew they were coming (visual) even though they didn't call, and just went for it.
It is sometimes difficult to see the start of the bridge close as the are often pretty open and until there are near the midpoint of their traverse, a small change on either end (horizontal or vertical) is difficult to perceive. Also, the skipper (you) are also trying to keep a course, often in a tight channel, and current through some bridges is fast, so steerage needs to be maintained, especially with the current. Focus on these things, draws your attention away from the start of a bridge closing until it is too late.
At some point you are fully committed and you just can't stop and brake. The first guy was at that point. The second guy just had enough time to change course. A second later, he would have been screwed.
We have to go under a few swing and draw bridges to/from our winter slumber yard, and it is always uneasy. Current through boat is often a little off angle to the narrow (30 - 40 ft) and 100 ft long slot. You have to go through at 5-6 kts else there is a change of hitting the walls from the cross current.
When going under/through a draw or swing bridge, I typically ask the bridge tender to open the bridge about 300-500 yards away. They respond with a wait time or say they the bridge will be open in 2-3 minutes. I continue a slow pace, just enough to keep steerage, towards the bridge. Once I see the bridge more than halfway open, I increase speed and move towards the bridge, calling back to the tender when I am about 200 ft away to announce that I am starting my pass under/through to let them know I am committing myself. Once fully through, I call back, say that I am safely through and thank them for their help.
May seem over talkative, but I'd rather have them know where I am at all times than for them to guess and start closing the bridge while I am transiting it.