I fly every week and have for years. For years passengers have 'jumped' up to make that last pit stop near the end of a flight.
You wrote the passengers went to the front lav. I infer they went from coach to first class -- a practice some airlines actually stop now, in the name of security, and perhaps a little privilege. If this was Southwest, or some airline like that, where there is no first class, then people I observe tend to go to the lav they can see.
If a group of passengers meeting a given demographic, ahem, went from the rear of the aircraft all the way to the forward lav near the end of the flight, I would say that was odd at best.
Also, it can be as simple as passengers logically noting that the rear lavs are occupied, and heading to the front.
My opinion: Passengers are no longer the threat. Between FAMs and veteran passengers, no hijacking attempt will ever be successful again on a US carrier.