We are really reaching into speculative territory here so I say this with the caveat that it is one of many possibilities…
The deadman on these cab cars is a foot switch. Many times the engineer drives while standing. He would need to stumble or fall to loose contact with the switch. If he “zoned out” as is said he told first responders, he wouldn’t necessarily loose balance and move off the switch.
A former director of the NTSB is a consultant to CBS Radio in NYC. He said the other day that he suspects the engineer may suffer from Sleep Apnea. As one who suffered undiagnosed with this for quite a few years, it really does make sense. Before I got treatment I was really never more than a few seconds away from being asleep. I had a very difficult time with a 2 hour highway commute and would often pull over two or three times during the trip to nap. It gets progressively worse over time and you don’t realize what the problem is because you think you are getting proper sleep.
As far as the rest of the crew goes, they may have been sleeping too. They were quite a few minutes out of their last stop on an express leg before the city. The crew has nothing to do at that point. The trains are very smooth. Several survivors have said they were sleeping and didn’t awake until the cars left the track.