The crowd of tormentors are leaving, becoming progressively less exotic in appearance as they reach their (very Dutch) towns and villages; their leaving highlights the isolation of the three principal personages.
(This shot and the master are official. Blurry ones below are mine. The blues are really very cool, as in the museum's photographs; mine are much too yellow.)
The first travelers were still talking about the event, even interacting with it on the picture plane. One of them, perhaps the one is the red hat, is presumably the centurion, forever changed by the experience, but identification is unclear. Those further away (on Christ's left hand) are resolutely leaving, faceless (though their spears are more pointedly aimed towards the nearly dead Christ).
Later the returning crowd melts into the day-to-day life of the background.
I'm very interested in the use of atmospheric perspective in this painting, which seems to connote more than mere distance: note how this cottage is lit in the same blue light, although it is in front of some still-green trees.
In the city behind, human figures are in full color. And look at the abrupt transition on the near side of the bridge! Below: another official detail, mostly for its beauty.
To give a totally spurious interpretation, i would suggest that the blue points to the overlap between the historical and the heavenly Jerusalem, one condemned to oblivion and the other created by the event shown in full color in the foreground. The real (walled cities and countrysides, more or less embellished) is rendered unreal. In either case, those who left the foreground can't fully return, are lifted from the distance in some way.
I do find this unsatisfactory, however, and the picture still appealingly puzzling.
Nothing, however, is as puzzling as this (to the left of the big rock; the wavy line above is Christ's loincloth). Is it the next step in the story? The cross does appear to bear no corpus. Or is it another, mundane crucifixion?