I'm not absolutely sure of the grammatical explanation, but I think that in this case "slouched" is used as an adjective, not a verb. In the present tense one could say "a couple of boys are slouched over the table" or "I saw a couple of boys slouched over the table".
You could just as well say "A couple of boys slouched over the table". The meaning is pretty much the same when used as a verb like this, depending on the context. The active verb form would be used if you wanted to talk about the action actually taking place: "A couple of boys sat down and slouched over the table." The adjective usage doesn't have the sense of immediacy; they may have been sitting there slouched over for the past hour.