if you push aquatic plants into the substrate, it damages the roots... to plant them you dig a hole in the sand, put them in, then cover the roots. DONT push or pull the plants in the sand, because the shells and grit will cut the outer layer of cells and the roots will die.
Even if only a few roots are damaged, it can starve the plant, so it helps to prune the plant back (if it has leaves, cut a few off, if it doesn't, then dont do anything unless you know what you're doing) so there is less demand on the remaining roots. in a normal plant, there is a balance between roots and stems, so if you damage some roots, it throws the balance off, and the stems die off to compensate. to counter this, you prune some stems to maintain the balance, and the plants stay healthy.
btw... this all holds true for terrestrial plants, too...