I didn't think he just happened to be present, maybe just happened to be present when they decided on a bigger flag, but my understanding is it was a bit of a hike to where they put the flag and nobody would just happen to be there.
I wouldn't say not everything is a photo-op, as you point out a good photographer can find the right shot in just about everything. I'd say that not all photo-ops have little pieces of tape on the floor (or a mysterious pile of stones on the beech as it were), and those tend to be the better ones.
They "happened to be there" because their units were there, and had been ordered to take the high ground so the Japs would quit lobbing stuff on everybody's heads.