I don't think it's by design. And Hall has thrown to enough WR's that I don't think he's favoring one over the other.
But it's funny how often catches seem to come in bunches.
Last night, it was Cosper getting three in a row in one drive and scoring the TD. In another game it was Roberts. It's not necessarily the heralded players who we expected to perform at the start of the season - but it seems like every game something like that happens, where one WR gets all the attention in a drive.