Romana gives them time, The Doctor and Jack, she talks to Mark and grieves for the dead.
Then, when she is stronger, focusing all her energy on making her feet move she moves across the ground leaving Mark to find Roger again.
Jack and Martha, they won't know her, they won't know she is still Romana, they may think her some interloper intruding on their grief, so she hangs back a little giving the Doctor time to focus himself.
He and Jack grasp each other in some kind of broken, hurt embrace and all she can do is watch them