I have been through Singapore airport over 150 times, on about 3 maybe 4 times there was security when getting off the plane. It is exactly the same security at the gate as you normally get when boarding. It is annoying as you have to line up again and take everything out of your pocket (otherwise how else are you going to get through the metal detector without it going PING). And repack your carry on after the x-ray. Annoying - but how can it be disrespectful when they are doing it to everyone on the plane?

The plane is also taken to the furthest arrival gate (which has multiple boarding gates, so they can run multiple x-ray and metal detectors at once, rather than the standard 2 per gate), none of the other boarding gates are used at this time, to keep the passengers on this plane separate, and get everyone off as quickly as possible. However, it does mean a long walk to where ever you want to go (arrivals, lounge, another gate etc.)

There is no way, NONE that when walking past all the other gates you saw every plane doing arrival security - departure YES, as security is done at the gate and not some central point, which makes it a great transit airport, but not arrival. As you hadn't been to Singapore before you wouldn't know that getting off a plane the air bridge leads straight into the concourse, not past x-rays.

They do this as they are looking for something specific. Or someone. There was some customs tip off and the one time you went to Singapore you were on a plane that likely had some drug couriers and you round on a country / airport that has a zero tolerance policy which the arrivals announcement would have said.

Ask questions on why do you think they did this on the one flight you arrived on rather than assuming it is normal.

And lastly - how can they tell that you as an arriving passenger is "Clearly" heading for arrivals and not transiting when all transiting and arriving passengers mix on a common concourse?