I noticed this myself and here's the explanation I came up with. I think the touching, either of the ground or the tree or the person's back(why couldn't Kakashi use the ground? ;0) ), is used for control purposes, when Naruto summoned Gamabunta he wasn't trying to summon Gamabunta specifically he was merely using the summon technique, remember Jiraiya was surprised when Naruto summoned him? This also was due to the enormous Kyuubi chakra Naruto used, but I think using a stable surface might be a way to control which summon comes out, ie. using the persons back for Kakashi to summon the small dog Pakkun. That's my idea, the other ones work too though, *shrugs*.