They may be hiding from you having your hand in there putting the new plants in. After they get used to me fiddling with the tank, my fish are always keen to inspect what I have trimmed, added or taken out.
Turn the lights out for a while and leave them be. They should find the plants and realise they are not a threat soon. Chook is right, they may be more scared and freightened of a humans hand and the change of structure in their tank. Id say give them a day or so and they should return to normal!