Shadows in this partially colourised pic from WW1 puzzle me. Shadows OF the men falling across the road at right-angle to officers'. How so?

This pic looks very much to me like it was photoshopped from two separate pics.

It is two photos that have been blended to look like one.
Notice in the color side the back-ground buildings are rather ordinary but the buildings in the B/W are war-torn.
It is a statement photo, the brightly colorised new soldiers listening to the war-weary older vets.

The men on the right have their shadow falling on a almost flat surface which is perpendicular to the way they are standing. While those on the left as standing of debris or something else and in front of a building. So of course the shadows would be different.

The sun is behind the soldiers' left shoulders.

What you're claiming is a ninety degree angle is actually a forty five degree angle, which the mass of enlisted men makes a sold mass.

Look at the individual helmet shadows, which all line up. Notice how their left chins and necks are in direct sunlight.

Insofar as the shadows, the shadows are aligned with the direction of the light. the half-finished colorization is simply still incomplete, obviously meaning the person did not finish what he/she started doing and we'll never know why since that person is probably long, long dead... unless the colorization is a modern colorization done through computer digital enhancing techniques.

Quite likely, in fact almost certainly, the part on the right is a recent posed photo. There was no practical form of colour photography which could be used "in the field" in WW1, it does not seem to be a "coloured up" part of a b/w photo. You can also see that the road surface varies between the left and the right side.